Method for producing organic light-emitting element

ABSTRACT

Method for manufacturing organic EL element including anode, hole injection layer, buffer layer, light-emitting layer, and cathode, layered on substrate in the stated order, and banks defining a light-emission region, and having excellent light-emission characteristics, due to the hole injection layer having excellent hole injection efficiency, being a tungsten oxide layer including an oxygen vacancy structure, formed under predetermined conditions to have an occupied energy level within a binding energy range from 1.8 eV to 3.6 eV lower than a lowest binding energy of a valence band, and after formation, subjected to atmospheric firing at a temperature within 200° C.-230° C. inclusive for a processing time of 15-45 minutes inclusive to have increased film density and improved dissolution resistance against an etching solution, a cleaning liquid, etc., used in a bank forming process.

TECHNICAL FIELD

The present invention relates to a method for manufacturing an organiclight-emitting element (referred to hereinafter as an “organic ELelement”), which is an electric light-emitting element. Particularly,the present invention relates to a technology for driving such anorganic EL element at low electricity while ensuring a wide range ofluminous intensity from low luminous intensity to high luminousintensity for the use as a light source or the like.

DESCRIPTION OF THE RELATED ART

In recent years, progress is being made in the research and developmentof various functional elements that utilize organic semiconductors.

An organic EL element is a typical example of such a functional element.An organic EL element is a current-driven light-emitting element, andtypically includes a pair of electrodes, composed of an anode and acathode, and one or more functional layers each containing organicmaterial disposed between the pair of electrodes. In the presentdisclosure, the term one or more functional layers refers to layers suchas a light-emitting layer and a buffer layer. In addition, a typicalorganic EL element may further include a hole injection layer disposedbetween the set of the one or more functional layer and the anode. Thehole injection layer has the function of injecting holes. In order todrive the organic EL element, voltage is applied across the pair ofelectrodes. This leads to holes being injected from the anode to the oneor more functional layers and electrons being injected from the cathodeto the one or more functional layers. Accordingly, the organic ELelement emits light by an electric-field light-emitting phenomenontaking place when the holes and the electrons recombine at the one ormore functional layers. An organic EL element exhibits high visibilityfor being self-luminescent and has high shock resistance for having afully solid-state structure. Due to such positive characteristics,organic EL elements are attracting much attention as light-emittingelements or as a light source in various display devices.

Organic EL elements can be largely divided into two types, dependingupon the material used for forming one or more functional layerstherein. The first type of organic EL elements is a vapor depositiontype. In a vapor deposition type organic EL element, the one or morefunctional layers are mainly composed of organic low molecular material,and film forming of the one or more functional layers is performed byapplying in-vacuum processing such as vapor deposition. The second typeof organic EL elements is an application type. In an application typeorganic EL element, the one or more functional layers is mainly composedof either organic high molecular material, organic low molecularmaterial having excellent thin-film forming property, etc., and filmforming of the one or more functional layers is performed by applyingwet processing such as the inkjet method and the gravure printingmethod.

When comparing the two types of organic EL elements, the development ofvapor deposition type organic EL elements has progressed to a furtherextent compared to the development of application type organic ELelements. Reasons for this include the higher light-emission efficiencyof the light-emission material included in vapor deposition type organicEL elements and the longer operating lifetime of vapor deposition typeorganic EL elements compared to application type organic EL elements(for example, refer to Patent Literatures 1 and 2). In fact, vapordeposition type organic EL elements have already been put into practicaluse in mobile phone displays and small-sized TVs, for example.

However, although vapor deposition type organic EL elements aredesirable for use in small-sized organic EL panels, the application ofvapor deposition type organic EL elements to, for example, full-color,large-sized organic EL panels having display sizes of around 100 inchesis extremely difficult. This is due to the technology used inmanufacturing vapor deposition type organic EL elements. Whenmanufacturing an organic EL panel by using vapor deposition type organicEL elements, the mask vapor deposition method is typically used forseparately forming light-emitting layers of different colors (here, thecolors of the light-emitting layers are R, G, and B, for example). Here,when the organic EL panel to be manufactured becomes greater in area, itis difficult to precisely adjust the position of the mask used in themask vapor deposition method due to reasons such as the difference inthermal expansion coefficients between the mask and a glass substrate.Hence, manufacturing of a display that operates properly is difficult.One possible countermeasure for overcoming such a problem is eliminatingthe necessity of separately forming, through application, thelight-emitting layers of different colors as described above by usingwhite-colored light-emitting layers over the entire surface of the paneland additionally providing color filters of the colors R, G, and B.However, when making such a configuration, the amount of light actuallyoutput from the panel is a mere third of the amount of light emitted bythe light-emitting layers. Thus, in principle, an increased amount ofpower is consumed by the panel.

In view of such problems of vapor deposition type organic EL elements,attempts are being made to realize the application of application typeorganic EL elements in manufacturing organic EL panels having largesizes. As already described above, in an application type organic ELelement, film forming of the one or more functional layers is performedby wet processing. The application of wet processing reduces thetechnical barrier in the manufacturing large-sized panels. This issince, when forming the one or more functional layers by performing wetprocessing, the precision when separately applying the one or morefunctional layers with respect to predetermined positions is basicallynot dependent upon substrate size.

In the meantime, much effort is also being made in the research anddevelopment of technology for enhancing light-emission efficiency oforganic EL elements. In order to cause an organic EL element to emitlight with high efficiently and high luminance but with low powerconsumption, it is important to increase the efficiency with whichcarriers (holes and electrons) are injected from the electrodes to theone or more functional layers. One typical yet effective measure forachieving the efficient injection of carriers to the one or morefunctional layers is to provide an injection layer between each of theelectrodes and the set of the one or more functional layers. Suchinjection layers are provided for reducing the energy barrier (injectionbarrier) in the injection of carriers to the one or more functionallayers. A hole injection layer, which is one type of such injectionlayers, is typically formed by using, for example, a film formed byvapor deposition of organic low molecular material such as copperphthalocyanine (CuPc), a film formed by application of a solution oforganic high molecular material such as PEDOT:PSS, or a film formed byvapor deposition, sputtering, etc., of inorganic material such asmolybdenum oxide. In particular, a report has been made that an organicEL element including a hole injection layer made of molybdenum oxide hasimproved hole injection efficiency to the one or more functional layersand has longevity (for example, refer to Patent Literature 3). Further,in a typical organic EL element, the hole injection layer is formed on asurface of an anode. The anode is typically made of a transparentconduction film of ITO, IZO, etc., a metal film of aluminum, etc., or ofa combination of such a transparent conduction film and such a metalfilm layered one on top of the other.

CITATION LIST Patent Literature

[Patent Literature 1]

-   Japanese Patent Publication No. 3369615    [Patent Literature 2]-   Japanese Patent Publication No. 3789991    [Patent Literature 3]-   Japanese Patent Application Publication No. 2005-203339

Non-Patent Literature

[Non-Patent Literature 1]

-   Jingze Li et al., Synthetic Metals 151, 141 (2005)    [Non-Patent Literature 2]-   Hiromi Watanabe et al., Yuki EL Tohronkai Dai 7 Kai Reikai    Yokoushuu, 17 (2008)    [Non-Patent Literature 3]-   Hyunbok Lee et al., Applied Physics Letters 93, 043308 (2008)    [Non-Patent Literature 4]-   Yasuo Nakayama et al., Yuki EL Tohronkai Dai 7 Kai Reikai Yokoushuu,    5 (2008)    [Non-Patent Literature 5]-   Kaname Kanai et al., Organic Electronics 11, 188 (2010)    [Non-Patent Literature 6]-   I. N. Yakovkin et al., Surface Science 601, 1481 (2007)

SUMMARY Technical Problem

However, there is a problem to be overcome in the manufacturing of theabove-described organic EL elements having the above-describedadvantages.

Specifically, when using tungsten oxide, which is material havingexcellent hole injection characteristics, as the material for the holeinjection layer in an organic EL element, after a film of tungsten oxideis formed on the substrate in the manufacturing of the organic ELelement, the film so formed is exposed to an etching solution, acleaning liquid, etc., during a bank forming process. When the tungstenoxide film, which is to become the hole injection layer, is exposed tothe etching solution, the cleaning liquid, etc., a problematic situationmay occur where portions of the tungsten oxide film dissolve to theetching solution, the cleaning liquid, etc., and thus the thickness ofthe tungsten oxide film is reduced at such portions (hereinafterreferred to as “film thickness reduction”).

When excessive film thickness reduction occurs, it becomes difficult toprovide the hole injection layer with necessary film thickness. Further,the roughness of the film surface of the hole injection layer increases,and the uniformity in film surface state over the entire hole injectionlayer also decreases. Accordingly, the occurrence of excessive filmthickness reduction may affect the hole injection characteristics of thehole injection layer.

In view of such problems, the present invention incorporates, in anorganic light-emitting element, a hole injection layer achieving bothexcellent hole injection characteristics and stability in a processperformed during mass production of organic EL panels.

Specifically, the present invention provides an organic light-emittingelement having a hole injection layer that has improved resistanceagainst the etching solution, the cleaning liquid, etc., used in thebank forming process (hereinafter referred to as “dissolutionresistance”) and excellent hole injection efficiency, and thus havingexcellent light-emission characteristics, and a manufacturing method forsuch an organic light-emitting element.

Solution to Problem

In view of the above, one aspect of the present invention is a methodfor manufacturing an organic light-emitting element, the methodcomprising: forming a tungsten oxide layer containing tungsten oxide ona base layer including an anode, the tungsten oxide layer formed to havean oxygen vacancy structure therein; firing the tungsten oxide layer;forming a film of barrier wall material on the fired tungsten oxidelayer; forming barrier walls by patterning the film of barrier wallmaterial in a predetermined pattern by using an etching solution, thebarrier walls defining an aperture; forming an organic layer within theaperture, the organic layer containing organic material; and forming acathode above the organic layer. In the method pertaining to one aspectof the present invention, in the firing of the tungsten oxide layer, thefiring is performed to provide the fired tungsten oxide layer withhigher dissolution resistance to the etching solution compared to thetungsten oxide layer, while ensuring that the fired tungsten oxide layerhas an oxygen vacancy structure therein.

Advantageous Effects of Invention

Through much devotion and consideration, the inventors of the presentinvention (hereinafter referred to as “present inventors”) have foundthat when formed under predetermined film forming conditions, an oxygenvacancy structure is formed in the tungsten oxide layer, which providesthe tungsten oxide layer with an occupied energy level bringing about animprovement in hole injection characteristics. In addition, the presentinventors have also found that, by firing the tungsten oxide layer soformed under predetermined, strictly-defined conditions, the dissolutionresistance of the tungsten oxide layer with respect to the etchingsolution, the cleaning liquid, etc., that are used during the bankforming process is improved, while ensuring that the tungsten oxidelayer retains the oxygen vacancy structure even after having undergonethe firing. Based on such findings, the present inventors have arrivedat one aspect of the present invention, which reduces an amount of thefilm thickness reduction and provides, as the hole injection layer, atungsten oxide layer having excellent hole injection characteristics.Thus, an organic light-emitting element manufactured according to themethod pertaining to one aspect of the present invention drives at lowvoltage and has excellent light-emission efficiency.

As already described above, the method pertaining to one aspect of thepresent invention reduces the film thickness reduction amount of thehole injection layer. Accordingly, when manufacturing an organic ELpanel in which a plurality of the organic light-emitting elementspertaining to one aspect of the present invention are disposed, theabsolute level of unevenness in terms of film thickness between the holeinjection layers included in the manufactured organic EL panel isreduced over the entire panel, and thus the ununiformity oflight-emission efficiency between the organic light-emitting elementsincluded in the manufactured organic EL panel is reduced.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-sectional view illustrating the structure ofan organic EL element pertaining to embodiment 1.

FIG. 2 is a schematic cross-sectional view illustrating the structure ofhole-only devices.

FIGS. 3A through 3C are graphs illustrating a dependence of drivingvoltages of hole-only devices on film forming conditions of a holeinjection layer.

FIG. 4 is a device characteristics diagram showing curves illustratingthe relation between applied voltages and current densities of thehole-only devices.

FIG. 5 is a device characteristics diagram showing curves illustratingthe relation between applied voltages and current densities of organicEL elements.

FIG. 6 is a device characteristics diagram showing curves illustratingthe relation between current densities and light-emission intensities ofthe organic EL elements.

FIG. 7 is a schematic cross-sectional view illustrating the structure ofa sample device for photoelectron spectroscopy measurement.

FIG. 8 illustrates a UPS spectrum of a tungsten oxide layer.

FIG. 9 illustrates UPS spectra of tungsten oxide layers.

FIG. 10 illustrates differential curves derived from the UPS spectraillustrated in FIG. 9.

FIG. 11 illustrates UPS spectra of tungsten oxide layers afteratmospheric exposure.

FIG. 12 illustrates a UPS spectrum and an XPS spectrum of a tungstenoxide layer pertaining to the present invention.

FIG. 13 illustrates an energetic state at an interface between thetungsten oxide layer pertaining to the present invention and an α-NPDlayer.

FIGS. 14A and 14B explain the effects of injection sites of a holeinjection layer and a functional layer.

FIG. 15 illustrates an energetic state at an interface between atungsten oxide layer formed under film forming conditions C and theα-NPD layer.

FIG. 16 is a graph illustrating the relation between film thicknessreduction amounts and driving voltages, for different film forming rateswhen forming tungsten oxide films.

FIG. 17 is a graph illustrating the relation between WOx film thicknessreduction amounts and driving voltages, for different film densities oftungsten oxide films.

FIG. 18 is a graph comparing UPS spectra of tungsten oxide films formedaccording to different film forming conditions.

FIG. 19 is a graph illustrating the relation between film densities andfilm thickness reduction amounts for different tungsten oxide films.

FIG. 20 explains the effects of introducing a firing process afterforming of a tungsten oxide film.

FIG. 21 is a graph illustrating the relation between film thicknessreduction amounts and panel surface film thickness unevenness fordifferent film densities of tungsten oxide films.

FIG. 22 is a graph illustrating the relation between film thicknessunevenness and current efficiency unevenness of tungsten oxide films.

FIG. 23 is a graph illustrating the relation between film thicknessreduction amounts and film densities, for different processing times ofthe firing process.

FIG. 24A is a schematic cross-sectional diagram illustrating thestructure of an organic EL element 1C pertaining to embodiment 2, andFIG. 24B is a partially expanded view near a hole injection layer 4A.

FIG. 25 is a schematic cross-sectional diagram illustrating thestructure of a hole-only device 1D.

FIGS. 26A through 26C each explain a process in a method formanufacturing the organic EL element 1C pertaining to embodiment 2.

FIGS. 27A and 27B each explain a process in the method for manufacturingthe organic EL element 1C pertaining to embodiment 2.

FIGS. 28A through 28C each explain a process in the method formanufacturing the organic EL element 1C pertaining to embodiment 2.

FIGS. 29A through 29D each explain a process in a method formanufacturing the organic EL element 1C pertaining to a modification ofembodiment 2.

FIGS. 30A through 30C each explain a process in the method formanufacturing the organic EL element 1C pertaining to the modificationof embodiment 2.

FIG. 31 is a device characteristics diagram showing curves illustratingthe relation between applied voltages and current densities of hole-onlydevices.

FIG. 32 is a device characteristics diagram showing curves illustratingthe relation between applied voltages and current densities of organicEL elements.

FIG. 33 is a diagram illustrating spectra belonging to W5p_(3/2),W4f_(5/2), and W4f_(7/2) obtained by HXPS measurement of tungsten oxidelayers.

FIG. 34A illustrates peak fitting analysis results for sample α in FIG.33, and FIG. 34B illustrates peak fitting analysis results for samplesin FIG. 33.

FIG. 35 illustrates UPS spectra of tungsten oxide layers.

FIG. 36 explains the structure of a tungsten trioxide crystal.

FIG. 37 illustrates cross-sectional TEM photographs of tungsten oxidelayers.

FIG. 38 illustrates 2D Fourier transform images for the TEM photographsshown in FIG. 37.

FIGS. 39A and 39B illustrate a process of creating a luminance varianceplot from a 2D Fourier transform image in FIG. 38.

FIG. 40 shows 2D Fourier transform images and luminance variance plotsfor samples α, β, and γ.

FIG. 41 shows 2D Fourier transform images and luminance variance plotsfor samples δ and ε.

FIG. 42 includes portions (a), (a1), and (a2) corresponding to sample αand portions (b), (b1), and (b2) corresponding to sample E, portions (a)and (b) each being a luminance variance plot, (a1) and (b1) each beingan enlarged diagram of a peak appearing closest to the center point inthe luminance variance plot in the corresponding one of (a) and (b), and(a2) and (b2) each being the first derivative of the luminance varianceplot in the corresponding one of (a1) and (b1).

FIG. 43A schematically illustrates hole conduction when a tungsten oxidelayer is formed mainly from a nanocrystal structure, and FIG. 43Bschematically illustrates hole conduction when a tungsten oxide layer isformed mainly from an amorphous structure.

FIG. 44 is a plan view illustrating a portion of an organic EL panelpertaining to a modification.

DETAILED DESCRIPTION

<Aspects of the Invention>

One aspect of the present invention is a method for manufacturing anorganic light-emitting element, the method comprising: forming atungsten oxide layer containing tungsten oxide on a base layer includingan anode, the tungsten oxide layer formed to have an oxygen vacancystructure therein; firing the tungsten oxide layer; forming a film ofbarrier wall material on the fired tungsten oxide layer; forming barrierwalls by patterning the film of barrier wall material in a predeterminedpattern by using an etching solution, the barrier walls defining anaperture; forming an organic layer within the aperture, the organiclayer containing organic material; and forming a cathode above theorganic layer. In the method pertaining to one aspect of the presentinvention, in the firing of the tungsten oxide layer, the firing isperformed to provide the fired tungsten oxide layer with higherdissolution resistance to the etching solution compared to the tungstenoxide layer, while ensuring that the fired tungsten oxide layer has anoxygen vacancy structure therein.

By forming a tungsten oxide layer under predetermined film formingconditions, an oxygen vacancy structure is formed in the tungsten oxidelayer, which provides the tungsten oxide layer with an occupied energylevel bringing about an improvement in the hole injectioncharacteristics. In addition, by firing the tungsten oxide layer soformed under predetermined, strictly-defined conditions, the dissolutionresistance of the tungsten oxide layer with respect to the etchingsolution, the cleaning liquid, etc., used during the bank formingprocess is improved, while ensuring that the tungsten oxide layerretains the oxygen vacancy structure even after having undergone thefiring. Accordingly, the film thickness reduction amount is reduced, anda tungsten oxide layer having excellent hole injection characteristicsis formed as a hole injection layer. As such, an organic light-emittingelement manufactured according to the method pertaining to one aspect ofthe present invention drives at low voltage and has excellentlight-emission efficiency.

In addition, the method pertaining to one aspect of the presentinvention reduces the film thickness reduction amount of the holeinjection layer. Accordingly, when manufacturing each of a plurality oforganic light-emitting elements included in an organic EL panelaccording to the method pertaining to one aspect of the presentinvention, the unevenness in film thickness between the hole injectionlayers included in the manufactured organic EL panel is reduced over theentire panel, and thus the ununiformity of light-emission efficiencybetween the organic light-emitting elements included in the manufacturedorganic EL panel is reduced.

In the method pertaining to one aspect of the present invention, in thefiring of the tungsten oxide layer, the firing may be performed toprovide the fired tungsten oxide layer with a film density of at least5.8 g/cm³ and at most 6.0 g/cm³.

In the method pertaining to one aspect of the present invention, in theforming of the tungsten oxide layer, the tungsten oxide layer may beformed to have an occupied energy level within a binding energy rangefrom approximately 1.8 electron volts to approximately 3.6 electronvolts lower than a lowest binding energy of a valence band, and in thefiring of the tungsten oxide layer, the firing may be performed whileensuring that the fired tungsten oxide layer has an occupied energylevel within the binding energy range.

In the method pertaining to one aspect of the present invention, in theforming of the tungsten oxide layer, the tungsten oxide layer may beformed such that at least one of an ultraviolet photoelectronspectroscopy spectrum and an X-ray photoelectron spectroscopy spectrumof the tungsten oxide layer exhibits an upward protrusion within abinding energy range from approximately 1.8 electron volts toapproximately 3.6 electron volts lower than a lowest binding energy of avalence band, and in the firing of the tungsten oxide layer, the firingmay be performed while ensuring that at least one of an ultravioletphotoelectron spectroscopy spectrum and an X-ray photoelectronspectroscopy spectrum of the fired tungsten oxide layer exhibits anupward protrusion within the binding energy range.

In the method pertaining to one aspect of the present invention, in theforming of the tungsten oxide layer, the tungsten oxide layer may beformed such that a differential spectrum obtained by differentiating anultraviolet photoelectron spectroscopy spectrum of the tungsten oxidelayer has a shape that is expressed by a non-exponential functionthroughout a binding energy range from approximately 1.8 electron voltsto approximately 3.6 electron volts lower than a lowest binding energyof a valence band, and in the firing of the tungsten oxide layer, thefiring may be performed while ensuring that a differential spectrumobtained by differentiating an ultraviolet photoelectron spectroscopyspectrum of the fired tungsten oxide layer has a shape that is expressedby a non-exponential function throughout the binding energy range.

In the method pertaining to one aspect of the present invention, in theforming of the tungsten oxide layer, the tungsten oxide layer may beformed to include tungsten atoms with a valence of six and tungstenatoms with a valence of five and thus have an oxygen vacancy structuretherein, a ratio W⁵⁺/W⁶⁺ of the number of the tungsten atoms with avalence of five to the number of the tungsten atoms with a valence ofsix being at least 3.2% and at most 7.4%, and in the firing of thetungsten oxide layer, the firing is performed while ensuring that theratio W⁵⁺/W⁶⁺ in the fired tungsten oxide layer is at least 3.2% and atmost 7.4%.

Another aspect of the present invention is a method for manufacturing anorganic light-emitting element, the method comprising: forming atungsten oxide layer on a base layer including an anode, the tungstenoxide layer containing tungsten oxide; firing the tungsten oxide layerto obtain a fired tungsten oxide layer; forming a film of barrier wallmaterial on the fired tungsten oxide layer; forming barrier walls bypatterning the film of barrier wall material in a predetermined patternby using an etching solution, the barrier walls defining an aperture;forming an organic layer within the aperture, the organic layercontaining organic material; and forming a cathode above the organiclayer. In the method pertaining to another aspect of the presentinvention, in the forming of the tungsten oxide layer, the tungstenoxide layer is formed such that at least one of an ultravioletphotoelectron spectroscopy spectrum and an X-ray photoelectronspectroscopy spectrum of the tungsten oxide layer exhibits an upwardprotrusion within a binding energy range from approximately 1.8 electronvolts to approximately 3.6 electron volts lower than a lowest bindingenergy of a valence band, and in the firing of the tungsten oxide layer,the firing is performed to provide the fired tungsten oxide layer withhigher dissolution resistance to the etching solution compared to thetungsten oxide layer, while ensuring that at least one of an ultravioletphotoelectron spectroscopy spectrum and an X-ray photoelectronspectroscopy spectrum of the fired tungsten oxide layer exhibits anupward protrusion within the binding energy range.

A yet another aspect of the present invention is a method formanufacturing an organic light-emitting element, the method comprising:forming a tungsten oxide layer on a base layer including an anode, thetungsten oxide layer containing tungsten oxide; firing the tungstenoxide layer to obtain a fired tungsten oxide layer; forming a film ofbarrier wall material on the fired tungsten oxide layer; forming barrierwalls by patterning the film of barrier wall material in a predeterminedpattern by using an etching solution, the barrier walls defining anaperture; forming an organic layer within the aperture, the organiclayer containing organic material; and forming a cathode above theorganic layer. In the method pertaining to yet another aspect of thepresent invention, in the forming of the tungsten oxide layer, thetungsten oxide layer is formed to include tungsten atoms with a valenceof six and tungsten atoms with a valence of five and thus have an oxygenvacancy structure therein, a ratio W⁵⁺/W⁶⁺ of the number of the tungstenatoms with a valence of five to the number of the tungsten atoms with avalence of six being at least 3.2% and at most 7.4%, and in the firingof the tungsten oxide layer, the firing is performed to provide thefired tungsten oxide layer with higher dissolution resistance to theetching solution compared to the tungsten oxide layer, while ensuringthat the ratio W⁵⁺/W⁶⁺ in the fired tungsten oxide layer is at least3.2% and at most 7.4%.

Each of the following embodiments includes description on an organic ELelement and a manufacturing method for an organic EL element,description on performance assessment experiments and observations.

Note that the drawings provide schematic illustration of components forthe sake of explanation, and thus, the illustration of the components inthe drawings may not be in accordance with their actual scale.

Embodiment 1

(Structure of Organic EL Element)

FIG. 1 is a schematic cross-sectional view illustrating the structure ofan organic EL element 1 pertaining to embodiment 1.

The organic EL element 1 is an application type organic EL element. Assuch, the organic EL element 1 includes one or more functional layershaving been formed by applying functional layer material by wetprocessing. The organic EL element 1 includes: a hole injection layer 4;one or more functional layers each containing organic material andhaving a predetermined function; and a pair of electrodes, composed ofan anode 2 and a cathode 8. The hole injection layer 4 and the set ofone or more functional layers, a buffer layer 6A and a light-emittinglayer 6B in this example, are disposed one on top of the other, and aredisposed between the pair of electrodes.

In specific, the organic EL element 1 includes, as illustrated in FIG.1, a substrate 10, and the anode 2, the hole injection layer 4, thebuffer layer 6A, the light-emitting layer 6B, and the cathode 8 (acombination of a barium layer 8A and an aluminum layer 8B) disposed inthe stated order on one main surface of the substrate 10. The anode 2and the cathode 8 are connected to a power supply indicated by DC inFIG. 1. Thus, power is supplied to the organic EL element 1 from theoutside.

(Substrate)

The substrate 10 is the base of the organic EL element 1. For example,the substrate 10 is made of insulating material such as alkali-freeglass, soda glass, nonfluorescent glass, phosphate glass, borate glass,quartz, acrylic resin, styrenic resin, polycarbonate resin, epoxy resin,polyethylene, polyester, silicone resin, and alumina.

While not illustrated in FIG. 1, at least one thin film transistor (TFT)for driving the organic EL element 1 is formed on the surface of thesubstrate 10.

(Anode)

The anode 2 is made of a 50 nm-thick transparent conductive film of ITO.However, the anode 2 is not limited to having such a structure. Forexample, the anode 2 may be made of: a transparent conductive film ofIZO or the like; a film of a metal such as aluminum; a film of an alloysuch as APC (an alloy of silver, palladium, and copper), ARA (an alloyof silver, rubidium, and gold), MoCr (an alloy of molybdenum andchromium) and NiCr (an alloy of nickel and chromium); or a combinationof a plurality of such films.

(Hole Injection Layer)

The hole injection layer 4 is a layer of tungsten oxide having a filmthickness of at least 2 nm (10 nm in this example). Further, in thecomposition formula (WOx) denoting the composition of tungsten oxidecontained in the hole injection layer 4, x is a real number existingwithin a range of approximately 2<x<3. The film thickness of the holeinjection layer 4 is set to at least 2 nm since the forming of a holeinjection layer with uniformity is difficult when the film thickness isset to less than 2 nm and also since it becomes difficult to form aSchottky ohmic contact between the hole injection layer 4 and the anode2 when the film thickness is set to less than 2 nm. Note that furtherdescription on the Schottky ohmic contact is provided later in thepresent disclosure. The Schottky ohmic contact is stably formed when thetungsten film is formed to have a film thickness of at least 2 nm.Therefore, by forming the hole injection layer 4 to have a filmthickness of at least 2 nm, the injection of holes from the anode 2 tothe hole injection layer 4 is performed stably via the Schottky ohmiccontact, and thus, the hole injection layer 4 is provided with excellenthole injection efficiency. Meanwhile, the hole injection layer 4 has afilm density within a range of at least 5.8 g/cm³ to at most 6.0 g/cm³.The hole injection layer 4 is provided with such a film density by afiring process being performed to densify the tungsten oxide containedin the hole injection layer 4. More specifically, after a tungsten oxidefilm is formed, the firing process is performed to fire and thus densifythe tungsten oxide film so formed. In the firing process, atmosphericfiring of the tungsten oxide film is performed under predeterminedconditions (at a firing temperature of at least 200° C. and at most 230°C., and for a processing time of at least 15 minutes to at most 45minutes). By performing the firing process, the film density of thetungsten oxide film, which is approximately within a range of at least5.4 g/cm³ to at most 5.7 g/cm³ immediately following the formingthereof, is increased to be within the above-described range of at least5.8 g/cm³ to at most 6.0 g/cm³. Increasing the film density of thetungsten oxide film in such a manner provides the tungsten oxide filmwith improved dissolution resistance to the etching solution, thecleaning liquid, etc., used in the bank forming process during themanufacturing of the organic EL element 1, and thus the film-thicknessreduction amount of the tungsten oxide film is suppressed to as small anamount as possible.

Note that as illustrated in FIG. 1, the hole injection layer 4 has adepression (hereinafter referred to as “concave portion”) that isconcave in the direction of the anode 2, at one surface thereof on theside of the light-emitting layer 6B. The concave portion is formed dueto a part of the surface on the side of the light-emitting layer 6Bbeing removed by being exposed to the etching solution, the cleaningliquid, etc., used in the bank forming process for forming banks 5.Here, it should be noted that the depth of the concave portion issmaller than the depth of the hole injection layer 4 at the bottom ofthe concave portion. As such, by the firing process being introduced inthe manufacturing of the organic EL element 1, the film thicknessreduction amount is suppressed by a considerable level compared to whenan organic EL element is manufactured according to conventionaltechnology. More specifically, the hole injection layer 4 has a filmthickness of at least 7 nm, which means that the hole injection layer 4,even when the film thickness reduction occurs, maintains afilm-thickness of at least 50% the film thickness (14 nm) of thetungsten oxide film immediately following the forming thereof.

Here, while it is desirable for the hole injection layer 4 to be made ofonly tungsten oxide, the inclusion of a trace level of impurities isacceptable, provided that the amount does not exceed the amount ofimpurities that might normally be incorporated.

The hole injection layer 4 is formed under specific film formingconditions. By forming the hole injection layer 4 under such specificfilm forming conditions, the hole injection layer 4 is provided with anoxygen vacancy structure. In the present disclosure, an oxygen vacancystructure refers to a structure where a tungsten atom is not bound tothe number of oxygen atoms regularly bound to the tungsten atom intungsten oxide. Further, due to having the oxygen vacancy structure, thehole injection layer 4 is provided, in an electronic state thereof, withan occupied energy level within the binding energy range from 1.8electron bolts (eV) to 3.6 eV lower than the lowest binding energy ofthe valence band. This occupied energy level corresponds to the highestoccupied energy level of the hole injection layer 4. Further, theoccupied energy level is closest to the Fermi level (Fermi surface) ofthe hole injection layer 4 in terms of binding energy. As such, thisoccupied energy level of the hole injection layer 4 is hereinafterreferred to as “occupied energy level near the Fermi surface”.

Note that the expression “occupied energy level” in the context of thepresent disclosure includes an energy level of a so-called semi-occupiedorbital, which is an electron orbital occupied by one electron.

Due to the existence of the occupied energy level near the Fermi surfacein the hole injection layer 4, a so-called interface energy levelalignment is formed at the layer interface between the hole injectionlayer 4 and the functional layer (the buffer layer 6A in this example).As such, the binding energy of the highest occupied molecular orbital(HOMO) of the buffer layer 6A and the binding energy of the occupiedenergy level near the Fermi surface of the hole injection layer 4 becomeapproximately equal.

Note that the expressions “approximately equal” and “interface energylevel alignment” as referred to herein indicate a state where thedifference between the lowest binding energy of the occupied energylevel near the Fermi surface of the hole injection layer 4 and thelowest binding energy of the HOMO of the functional layer, at aninterface between the hole injection layer 4 and the functional layer,is within a range of ±0.3 eV inclusive.

Furthermore, the expression “interface” in this case refers to an areathat includes a surface of the hole injection layer 4 and a portion ofthe buffer layer 6A within a distance of 0.3 nm from the surface of thehole injection layer 4.

In addition, while it is desirable that the occupied energy level nearthe Fermi surface exist throughout the hole injection layer 4, itsuffices for the occupied energy level near the Fermi surface to existat least at the interface with the buffer layer 6A. Further, it shouldbe noted that not all tungsten oxide has the occupied energy level nearthe Fermi surface; rather, the occupied energy level near the Fermisurface is a unique energy level that is formed within the holeinjection layer 4 and at the interface with the buffer layer 6A onlywhen forming the hole injection layer 4 under the predetermined filmforming conditions described below.

Additionally, the hole injection layer 4 is characterized for formingthe so-called Schottky ohmic contact at the interface with the anode 2.

The expression “Schottky ohmic contact” as referred to herein denotesthat the difference between the Fermi level of the anode 2 and theabove-described lowest binding energy of the occupied energy level nearthe Fermi surface of the hole injection layer 4 is relatively small,namely within a range of ±0.3 eV inclusive, at a position that is 2 nmaway from the surface of the anode 2 towards the hole injection layer 4.Furthermore, the expression “interface” in this case refers to a regionthat includes a surface of the anode 2 and the Schottky barrier formedaway from the surface towards the hole injection layer 4.

Meanwhile, through much consideration, the present inventors have foundthat the dissolution resistance of a tungsten oxide film against theetching solution, the cleaning liquid, etc., used in the bank formingprocess increases proportionally as the film density of the tungstenoxide film increases. In addition, the present inventors also have foundthat the hole injection characteristics of a tungsten oxide filmdecrease inverse-proportionally as the film density of the tungstenoxide film increases. That is, the present inventors have found thatthere is a trade-off relation between the hole injection characteristicsand the dissolution resistance of the tungsten oxide film. In view ofthis, the hole injection layer 4 pertaining to embodiment 1 realizesboth excellent hole injection characteristics and high dissolutionresistance at the same time and at a high level. This is realized by (i)providing the hole injection layer 4 with the occupied energy level nearthe Fermi surface by forming the tungsten oxide film according to thepredetermined film forming conditions and (ii) providing the holeinjection layer 4 with increased dissolution resistance by increasingthe film density of the tungsten oxide film by performing a firingprocess according to predetermined, strictly-defined conditions afterforming the tungsten oxide film.

In addition, the present invention reduces the film thickness reductionamount of the hole injection layer 4 as described above. Due to this,when forming an organic EL panel by disposing a plurality of the organicEL elements 1, the unevenness in film thickness between the holeinjection layers 4 in the organic EL panel manufactured is suppressedover the entire panel. Thus, the ununiformity of light-emissionefficiency between the plurality of organic EL elements 1 is reduced.

(Banks)

On the surface of the hole injection layer 4, banks (barrier walls) 5are formed. The banks 5 define the area of the light-emitting layer 6B.The banks 5 are made of organic material having insulating property (forexample, acrylic resin, polyimide resin, or novolac type phenolicresin). The banks 5 are formed so as to have uniform trapezoidalcross-sections, and so as to form either a line bank structure or apixel bank structure.

Here, it should be noted that the banks 5 are not indispensable in thepresent invention, and need not be formed when the organic EL element 1is to be used by itself.

(Functional Layer)

On a portion of the surface of the hole injection layer 4 defined by thebanks 5, the one or more functional layers, composed of the buffer layer6A and the light-emitting layer 6B in this example, are formed. Here,the light-emitting layer 6B corresponds to one of the colors R, G, andB. Each of the one or more functional layers is an organic layercontaining organic material. Further, when forming an organic EL panelby using a plurality of the organic EL elements 1, a plurality of pixelunits each composed of three, sequentially arranged organic EL elements1 for the colors R, G, and B are arranged in an array on the substrate10.

(Buffer Layer)

The buffer layer 6A is a layer that efficiently transports holes fromthe hole injection layer 4 to the light-emitting layer 6B. The bufferlayer 6A is a 20 nm-thick layer made of TFB(poly(9,9-di-n-octylfluorene-alt-(1,4-phenylene-((4-sec-butylphenyl)imino)-1,4-phenylene)),which is an amine-containing organic high molecular material.

(Light-Emitting Layer)

The light-emitting layer 6B is a 70 nm-thick layer made of F8BT(poly(9,9-di-n-octylfluorene-alt-benzothiadiazole)), which is an organichigh molecular material. However, the light-emitting layer 6B is notlimited to being made of such material, and the light-emitting layer 6Bmay include a commonly-known organic material. Examples of such acommonly-known organic material that may be included in thelight-emitting layer 6B include fluorescent material, such as an oxinoidcompound, perylene compound, coumarin compound, azacoumarin compound,oxazole compound, oxadiazole compound, perinone compound,pyrrolo-pyrrole compound, naphthalene compound, anthracene compound,fluorene compound, fluoranthene compound, tetracene compound, pyrenecompound, coronene compound, quinolone compound and azaquinolonecompound, pyrazoline derivative and pyrazolone derivative, rhodaminecompound, chrysene compound, phenanthrene compound, cyclopentadienecompound, stilbene compound, diphenylquinone compound, styryl compound,butadiene compound, dicyanomethylene pyran compound, dicyanomethylenethiopyran compound, fluorescein compound, pyrylium compound,thiapyrylium compound, selenapyrylium compound, telluropyryliumcompound, aromatic aldadiene compound, oligophenylene compound,thioxanthene compound, anthracene compound, cyanine compound, acridinecompound, metal complex of an 8-hydroxyquinoline compound, metal complexof a 2-bipyridine compound, complex of a Schiff base and a group threemetal, metal complex of oxine, rare earth metal complex, etc., asdisclosed in Japanese Patent Application Publication No. H5-163488.

Note that in the present disclosure, the one or more functional layersmay refer to either one of, a combination of more than two of, or acombination of all of layers such as a hole transfer layer, alight-emitting layer, and a buffer layer. A hole transfer layertransfers holes, a light-emitting layer emits light as a result ofrecombination of holes and electrons which are injected thereto, and abuffer layer is used for adjusting optical characteristics and/or forblocking electrons. Further, a typical organic EL element may include,in addition to a hole injection layer, a layer performing the respectivefunctions of the above-described hole transfer layer, light-emittinglayer, and the like. As such, the term one or more functional layers inthe present disclosure refers to one or more layers, excluding the holeinjection layer, that need to be included in the organic EL element 1and that are disposed between the anode 2 and the light-emitting layer6B.

(Cathode)

The cathode 8 is formed by layering a 5 nm-thick barium layer 8A and a100 nm-thick aluminum layer 8B one on top of the other.

Note that an electron transport layer may be disposed between thelight-emitting layer 6B and the cathode 8. Further, the barium layer 8Amay be considered as being an electron transport layer (or an electroninjection layer).

(Effects of Organic EL Element)

In the organic EL element 1 having the structure described above, thehole injection layer 4 is provided with the oxygen vacancy structure,and thus the hole injection layer 4 has the occupied energy level nearthe Fermi surface. Further, a so-called interface energy level alignmentis formed between the occupied energy level near the Fermi surface andthe HOMO of the buffer layer 6A, thereby reducing the hole injectionbarrier between the hole injection layer 4A and the buffer layer 6A toan extremely low level.

In addition to the above, in the organic EL element 1, excellentSchottky ohmic contact is formed between the anode 2 and the holeinjection layer 4, thereby suppressing the hole injection barrierbetween the anode 2 and the hole injection layer 4 to a low level.

Hence, when voltage is applied for driving the organic El element 1,holes are smoothly injected, even at low voltage, from the Fermi surfaceof the anode 2 to the occupied energy level near the Fermi surface ofthe hole injection layer 4, and from the occupied energy level near theFermi surface of the hole injection layer 4 to the HOMO of the bufferlayer 6A. As such, the organic EL element 1 has excellent hole injectionefficiency. Due to this, when the holes so injected arrive at thelight-emitting layer 6B and recombine with electrons at thelight-emitting layer 6B, the organic EL element 1 exhibits excellentlight-emission characteristics. More specifically, the difference inbinding energy between the Fermi level of the anode 2 and the lowestbinding energy of the occupied energy level near the Fermi surface ofthe hole injection layer 4 and the difference in binding energy betweenthe lowest binding energy of the occupied energy level near the Fermisurface of the hole injection layer 4 and the lowest binding energy ofthe HOMO of the buffer layer 4A are both within the range of ±0.3 eVinclusive, and thus, the hole injection efficiency of the organic ELelement 1 is enhanced to an extremely high level.

In addition, the Schottky ohmic contact between the anode 2 and the holeinjection layer 4 is highly stable not affected much, by the surfacecondition of the anode 2 (including characteristics of the surface suchas the work function thereof). Due to this, there is no need ofcarefully controlling the surface condition of the anode 2 during themanufacturing of the organic EL element 1. As such, the organic ELelement 1, which has excellent hole injection efficiency, and alarge-size organic EL panel including a plurality of the organic ELelements 1 formed therein are manufacturable at a low cost and with ahigh yield.

Here, note that the surface condition of the anode 2 specifically refersto the surface condition of the anode 2 immediately before the formingof the hole injection layer 4, in a typical manufacturing procedure ofan organic EL element or an organic EL panel.

In addition, in the manufacturing of the organic EL element 1, thefilm-density of the hole injection layer 4 is increased. Due to this,the hole injection layer 4 is provided with high dissolution resistanceand the film thickness reduction amount of the hole injection layer 4 isreduced. Meanwhile, by forming the hole injection layer 4 under thepredetermined, strictly defined film forming conditions, the holeinjection layer 4 is provided with the occupied energy level near theFermi surface. Thus, the hole injection layer 4 exhibits excellent holeinjection characteristics, which further leads to effectively reducingthe driving voltage of the organic EL element 1.

Note that a report has been made in the past of the technology of usingtungsten oxide as the material for a hole injection layer (refer toNon-Patent Literature 1). However, the hole injection layer obtained inthis report has an optimum film thickness of approximately 0.5 nm, andfurther, element characteristics were greatly dependent upon filmthickness. Thus, Non-Patent Literature 1 does not disclose technologyhaving a level of practicality which enables mass production oflarge-sized organic EL panels. Furthermore, Non-Patent Literature 1 doesnot disclose deliberately forming the occupied energy level near theFermi surface in a hole injection layer. In contrast, the presentinvention provides the predetermined occupied energy level near theFermi surface to a hole injection layer made of tungsten oxide, whichhas chemical stability and withstands processing during mass productionof large-sized organic EL panels. The provision of the occupied energylevel near the Fermi surface to the hole injection layer realizesexcellent hole injection efficiency of the hole injection layer andenables the organic EL element to be driven at low voltage. In addition,the present invention further distinguishes over such conventionaltechnology for providing the hole injection layer with increaseddissolution resistance and thereby ensuring that the excellent holeinjection characteristics of the hole injection layer are stablymaintained.

In the following, description is provided of an example of a method formanufacturing the entire organic EL element 1.

(Method for Manufacturing Organic EL Element)

Firstly, the substrate 10 is mounted inside a chamber of a sputteringfilm forming device. Then, a predetermined gas is introduced into thechamber, and the anode 2, having a thickness of 50 nm and made of ITO,is formed by reactive sputtering.

Subsequently, the hole injection layer 4 is formed on a base layerincluding the anode 2. The hole injection layer 4 is made of a tungstenoxide film containing tungsten oxide having an oxygen vacancy structure.In this example, the hole injection layer 4 is formed directly on asurface of the anode 2. Here, it is desirable that the hole injectionlayer 4 be formed by reactive sputtering. Especially, when the organicEL element 1 is to be used in a large-sized organic EL panel, wherethere is a need of forming a plurality of the hole injection layers 4over a large area, the forming of the hole injection layer 4 by vapordeposition is problematic in that there is a risk of unevenness in filmthickness, etc., occurring. However, by forming the hole injection layer4 by reactive sputtering, the occurrence of such unevenness in theforming of the hole injection layer 4 can be readily prevented.

In specific, reactive sputtering is performed after replacing thesputtering target with metal tungsten. Further, argon gas and oxygen gasare respectively introduced into the chamber as the sputtering gas andthe reactive gas. Under this condition, application of high voltage isperformed, whereby the argon in the argon gas is ionized, and theionized argon is caused to bombard the sputtering target. The metaltungsten ejected as a result of the sputtering phenomenon reacts withthe oxygen gas, and produces tungsten oxide. As a result, a tungstenoxide film is formed on the anode 2, above the substrate 10.

In the present disclosure, note that the predetermined film formingconditions under which the tungsten oxide film (the hole injection layer4) is formed are also referred to as “low rate” conditions. Typically, afilm forming rate when forming a film (i.e., a deposition rate) is setby controlling both an input power density of a film forming device anda ratio of a flow amount of a gas to the total flow amount of thechamber gas (partial pressure of the gas) in the film forming device. Inspecific, when forming a tungsten oxide film, the film forming rate canbe decreased by increasing the flow amount of oxygen gas in the chambergas (i.e., by increasing the partial pressure of the oxygen gas).Desirably, the specific “low rate” conditions are film formingconditions such that: a pressure of the gas introduced into thesputtering device (a total pressure of gas) is greater than 2.7 pascals(Pa) and at most 7.0 Pa; a partial pressure of the oxygen gas is atleast 50% and at most 70% of the total pressure of gas; and an inputpower density per unit area of the sputtering target is at least 1 W/cm²and smaller than 2.8 W/cm². By being formed under the low rate, thetungsten oxide film has porousness close to that of a vapor depositionfilm. Further, the tungsten oxide film has an oxygen vacancy structureformed at least at a surface portion thereof, and thus an occupiedenergy level within a binding energy range from 1.8 eV to 3.6 eV lowerthan the lowest binding energy of the valence band. As such, it isensured that the hole injection layer 4 has excellent hole injectioncharacteristics.

Subsequently, a firing process is performed with respect to the tungstenoxide film so formed. Specifically, atmospheric firing of the tungstenoxide film is performed under predetermined conditions, i.e., at afiring temperature of at least 200° C. and at most 230° C., and for aprocessing time of at least 15 minutes to at most 45 minutes. In thefiring process, care should be taken concerning the firing temperature,since when an interlayer insulating film (planarizing film), etc., aredisposed on the surface of the substrate 10, there is a risk of suchfilms undergoing degradation if the firing temperature is too high.

This firing process is performed to apply heat and thereby harden anddensify the tungsten oxide film. Specifically, by conducting the firingprocess, the film density of the tungsten oxide film, which is within arange of at least 5.4 g/cm³ and at most 5.7 g/cm³ immediately afterforming, changes (increases) to be within a range of at least 5.8 g/cm³and at most 6.0 g/cm³. In addition, by performing the firing processaccording to the predetermined conditions described above, the oxygenvacancy structure existing within the film is retained. As such, thetungsten oxide film retains the occupied energy level near the Fermisurface even after the firing process is performed, and thus, there isno concern of the hole injection characteristics of the hole injectionlayer 4 being deteriorated by the firing process. Further, since thefilm density of the tungsten oxide film is increased by conducting thefiring process, the dissolution resistance of the tungsten oxide filmwith respect to the etching solution, the cleaning liquid, etc., to beused for processing bank material as later described is increased to atleast twice the level at the point immediately after the forming of thetungsten oxide film. Thus, the film thickness reduction amount of thetungsten oxide film is effectively reduced.

The hole injection layer 4 is formed by performing such processes.

Subsequently, as the material for forming the banks 5, photosensitiveresin material for example, or more desirably, photoresist materialcontaining fluorine material is prepared. The bank material so preparedis uniformly applied on the hole injection layer 4, and prebaking isperformed. Subsequently, a mask having an opening of a predeterminedshape that is in accordance with the pattern of the banks 5 to be formedis placed over the bank material film having been formed. After exposingthe bank material film to light from over the mask, patterning of thebank material film is performed by washing away unhardened, redundantbank material by using a developer (the etching solution). Here, aconventional etching solution may be used, such as a tetramethylammoniumhydroxide (TMAH) solution. After the etching is completed, cleaning byusing the cleaning liquid (pure water, for example) is performed tocomplete the forming of the banks 5.

As described above, the firing process is performed to densify the holeinjection layer 4 in embodiment 1. Due to this, the hole injection layer4 exhibits a certain level of dissolution resistance with respect toalkaline solutions, water, organic solvents, etc. As such, even when thehole injection layer 4 falls in contact with the etching solution, purewater, etc., during the bank forming process, the occurrence of the filmthickness reduction of the hole injection layer 4 due to dissolution tothe etching solution, the cleaning liquid, etc., is suppressed comparedto a tungsten oxide film with respect to which the firing process hasnot been performed. Thus, the hole injection layer 4 is maintained inappropriate form until completion of the manufacturing of the organic ELelement 1. This too enables the efficient injection of holes via thehole injection layer 4 to the buffer layer 6A after the manufacturing ofthe organic EL element 1 is completed, and thus realizes excellent lowvoltage drive of the organic EL element 1.

Following this, the buffer layer 6A is formed by depositing drops of inkcomposition containing organic amine-containing molecular material ontoa surface of the hole injection layer 4 that is exposed from betweenadjacent ones of the banks 5 and by removing the solvent of the inkcomposition by volatilization. Here, the application of ink compositionis performed by wet processing such as the inkjet method and the gravureprinting method. Thus, the buffer layer 6A is formed.

Next, drops of ink composition containing organic light-emissionmaterial are deposited onto the surface of the buffer layer 6A, and thesolvent of the ink composition is removed by volatilization. Here, notethat the application of ink composition is performed according to amethod similar to that used in the forming of the buffer layer 6A. Thus,the light-emitting layer 6B is formed.

Here, it should be noted that the method applied for forming the bufferlayer 6A and the light-emitting layer 6B is not limited to theabove-described method. Other conventional methods besides the inkjetmethod and the gravure printing method may be used in the deposition andapplication of ink. Such conventional methods include: the dispensermethod; the nozzle coating method; the spin coating method; intaglioprinting; and relief printing.

Subsequently, the barium layer 8A and the aluminum layer 8B are formedon a surface of the light-emitting layer 6B by vacuum vapor deposition.Thus, the cathode 8 is formed.

Note that although not illustrated in FIG. 1, a sealing layer may beadditionally provided on the surface of the cathode 8, or a sealing capmay be provided to isolate the entire organic EL element 1 from externalspace, in order as to prevent atmospheric exposure of the organic ELelement 1. Such a sealing layer as described above is formed, forexample, by using material such as silicon nitride (SiN) and siliconoxynitride (SiON), and is disposed such that the organic EL element 1 issealed therein. Such a sealing cap as described above is formed byusing, for example, the same material as the substrate 10, and a getterwhich absorbs moisture and the like is provided within the sealed spaceformed by the substrate 10 and the sealing cap.

Through the above-described processes, the organic EL element 1 ismanufactured.

<Experiments and Observations>

(Film Forming Conditions of Tungsten Oxide Film)

In embodiment 1, due to being formed under the predetermined filmforming conditions, the tungsten oxide film which constitutes the holeinjection layer 4 is provided with the occupied energy level near theFermi surface. The occupied energy level near the Fermi surface reducesthe hole injection barrier between the hole injection layer 4 and thebuffer layer 6A, and thereby enables the organic EL element 1 to driveat low voltage.

A tungsten oxide film realizing the above-described performance of theorganic EL element 1 is desirably formed by performing reactivesputtering in a DC magnetron sputtering device, by using metal tungstenas the sputtering target, not performing any control of substratetemperature, and using a chamber gas composed of argon gas and oxygengas, and further, by performing the reactive sputtering under thepredetermined film forming conditions such that: the pressure of the gasintroduced into the sputtering device (the total pressure of gas) isgreater than 2.7 Pa and at most 7.0 Pa; the partial pressure of theoxygen gas is at least 50% and at most 70% of the total pressure of gas;and the input power density per unit area of the sputtering target is atleast 1 W/cm² and smaller than 2.8 W/cm².

The effectiveness of such film forming conditions has been provedthrough the following experiments.

Firstly, the present inventors prepared hole-only devices as assessmentdevices to be used in accurately determining the dependence of holeinjection efficiency upon film forming conditions. Needless to say, herethe hole injection efficiency refers to the efficiency with which holesare injected from the hole injection layer 4 into the buffer layer 6A.

Note that in an organic EL element, electric current is typically formedof carriers consisting of both holes and electrons. As such, theelectrical characteristics of an organic EL element reflects electroncurrent as well as hole current. However, in hole-only devices such asthose prepared by the present inventors, the injection of electrons fromthe cathode is blocked and thus there is almost no flow of electroncurrent. As such, electrical current flowing in the hole-only devicesconsists almost entirely of hole current, and it could be consideredthat only holes function as carriers in such hole-only devices. This iswhy, the hole-only devices are desirable for assessing hole injectionefficiency.

In specific, the present inventors prepared hole-only devices eachdiffering from the organic EL element 1 illustrated in FIG. 1 in thatthe cathode 8 is replaced with a cathode 8C made of gold (Au), asillustrated in FIG. 2. More specifically, as illustrated in FIG. 2, eachhole-only device includes a substrate 10 and a 50 nm-thick anode 2 madeof an ITO thin film and formed on the substrate 10, and further includesthe following layers disposed above the anode 2 in the stated order: a30 nm-thick hole injection layer 4 made of tungsten oxide; a 20 nm-thickbuffer layer 6A made of TFB, which is an organic amine-containingorganic high molecular material; a 70 nm-thick light-emitting layer 6Bmade of F8BT, which is an organic high molecular material; and a 100nm-thick cathode 8C made of gold. Note that, considering that thehole-only devices are assessment devices, the banks 5 were not includedtherein.

In the manufacturing of the hole-only devices, the hole injection layers4 were formed by reactive sputtering in a DC magnetron sputteringdevice. The chamber gas was composed of at least one of argon gas andoxygen gas, and metal tungsten was used as the sputtering target.Further, no control was performed of substrate temperature, whilecontrol of the partial pressure of the argon gas, the partial pressureof the oxygen gas, and the total pressure of gas was performed bycontrolling the flow amount of the gases. Further, the hole injectionlayer 4 in each of the hole-only devices was formed according todifferent film forming conditions. Here, the film forming conditionsdiffer from each other in terms of the total pressure of gas, thepartial pressure of the oxygen gas, and input power, as illustrated inTable 1. As a result, hole-only devices 1B (devices No. 1 through No.14) having hole injection layers 4 formed under different film formingconditions were obtained. Note that, hereinafter, the partial pressureof the oxygen gas is indicated in percentage (%) with respect to thetotal pressure of the chamber gas.

TABLE 1 Film Forming Conditions of the Hole-only Devices 1B Device No. 12 3 4 5 6 7 Oxygen 70 50 100 50 70 100 70 Partial Pressure (%) Total 2.74.8 1.7 1.7 2.7 1.7 2.7 Pressure (Pa) Input 500 500 500 500 250 250 1000Power (W) T-S (mm) 113 113 113 113 113 113 113 Film 0.164 0.14 0.1110.181 0.057 0.308 Forming Rate (nm/s) Film 30 30 30 30 30 30 30Thickness (nm) Device No. 8 9 10 11 12 13 14 Oxygen 50 100 50 70 30 3050 Partial Pressure (%) Total 4.8 1.7 2.7 1.7 1.7 2.7 4.8 Pressure (Pa)Input 1000 1000 500 500 500 500 250 Power (W) T-S (mm) 113 113 113 113113 113 113 Film 0.311 0.246 0.154 0.153 0.364 0.177 0.049 Forming Rate(nm/s) Film 30 30 30 30 30 30 30 Thickness (nm)

Table 2 illustrates the relation between input power and input powerdensity of the DC magnetron sputtering device.

TABLE 2 Input Power Input Power Density (W) (W/cm²) 250 1.4 500 2.8 10005.6

Each of the hole-only devices 1B so prepared was connected to the directcurrent power supply DC, and voltage was applied thereto. Here,different voltages were applied to the hole-only devices 1B forexperimentation, and current values for different voltage values weremeasured. Further, the current values were converted into current valuesper unit surface area of the devices (current densities). Note thathereinafter, the expression “driving voltage” refers to applied voltagefor a current density of 10 mA/cm².

Here, assumption is made that the lower the driving voltage of ahole-only device, the greater the hole injection efficiency from thehole injection layer 4 to the buffer layer 6A in the hole-only device.This is since, parts of the hole-only devices 1B other than the holeinjection layer 4 were prepared in the same way, and thus, assumption ismade that the hole injection barrier between two adjacent layers,excluding the hole injection layer 4, is uniform in each of thehole-only devices 1B. Furthermore, as described below, it was confirmedthrough another experiment that Schottky ohmic contact is formed betweenthe anode 2 and the hole injection layer 4 in each of the hole-onlydevices 1B used in this experiment. For the above reasons, thedifferences observed in the hole-only devices 1B, which are due to thedifferent film forming conditions, are considered to strongly reflectthe hole injection efficiency from the hole injection layer 4 to thebuffer layer 6A in the hole-only devices 1B and the hole conductionefficiency of the hole injection layer 4 itself in the hole-only devices1B.

As described above, assumption is made that, in addition to the holeinjection efficiency from the hole injection layer 4 to the buffer layer6A, the hole conduction efficiency of the hole injection layer 4influences the characteristics of the hole-only devices 1B used in theexperiments in embodiment 1. From the assessment of energy diagramsdescribed below, it is at least evident that the hole injection barrierbetween the hole injection layer 4 and the buffer layer 6A definitelyand strongly influences the characteristics of the hole-only devices 1B.

Further, note that in embodiment 1, results of observation concerningthe hole injection efficiency from the hole injection layer 4 to thebuffer layer 6A are mainly provided, whereas in embodiment 2, results ofobservation concerning the hole conduction efficiency of the holeinjection layer 4 are mainly provided.

Table 3 illustrates driving voltage values of the hole-only devices 1Bin relation to different film forming conditions, namely differentvalues of the total pressure of the chamber gas, the partial pressure ofthe oxygen gas, and input power. Note that the numbers enclosed incircles in Table 3 indicate device numbers of the hole-only devices 1B.

TABLE 3 Film Forming Conditions and Driving Voltages of the Hole-onlyDevices 1B (Applied Voltage Value under Electric Current Density of 10mA/cm²) Total Pressure 1.7 Pa 2.7 Pa 4.8 Pa Oxygen  30% {circle around(12)}500 W {circle around (13)}500 W (19 V) Film not formed Partial(Unmeasured) Pressure  50% {circle around (4)} 500 W {circle around(10)}500 W (19 V) {circle around (14)}250 W (13.7 V) (19 V) {circlearound (2)} 500 W (13.7 V) {circle around (8)} 1000 W (>20 V)  70%{circle around (11)}500 W {circle around (5)} 250 W (14.7 V) Film notformed (Unmeasured) {circle around (1)} 500 W (18.2 V) {circle around(7)} 1000 W (>20 V) 100% {circle around (6)} 250 W Film not formed Filmnot formed (Unmeasured) {circle around (3)} 500 W (>20 V) {circle around(9)} 1000 W (>20 V) *Numbers enclosed in circles indicate device No.,numbers without parenthesis indicate input electricity, and numbersplaced in parenthesis indicate driving voltage.

Further, FIGS. 3A through 3C are graphs summarizing the dependence ofdriving voltages of the hole-only devices 1B on different film formingconditions. Each of the points in FIG. 3A indicates, from left to right,a driving voltage of devices No. 4, 10, and 2. Similarly, each of thepoints in FIG. 3B indicates, from left to right, a driving voltage ofthe devices No. 13, 10, and 1. Finally, each of the points in FIG. 3Cindicates, from left to right, a driving voltage of the devices No. 14,2, and 8.

Note that in the present embodiment, the forming of the hole injectionlayer 4 was not performable under specific film forming conditions dueto limitations on the flow amount of gas, etc., on the side of thesputtering device. Specifically, the forming of the hole injection layer4 was not performable for each of the following film forming conditions:(i) when the total pressure of gas is 2.7 Pa and the partial pressure ofthe oxygen gas is 100%; (ii) when the total pressure of gas is 4.8 Paand the partial pressure of the oxygen gas is 30%; (iii) when the totalpressure of gas is 4.8 Pa and the partial pressure of the oxygen gas is70%; and (iv) when the total pressure of gas is 4.8 Pa and the partialpressure of the oxygen gas is 100%.

Firstly, the dependence of driving voltage on the total pressure ofchamber gas is analyzed by referring to experiment results for hole-onlydevices 1B formed with the partial pressure of the oxygen gas being setto 50% and the input power being set to 500 W (devices No. 4, 10, and2). When comparing such hole-only devices 1B, an obvious decrease indriving voltage was observed at least while the total pressure of gaswas within a range of greater than 2.7 Pa and at most 4.8 Pa, asillustrated in FIG. 3A. In addition, the present inventors observedthrough another experiment that this tendency of decrease in drivingvoltage continues at least until the total pressure of gas equals 7.0Pa. Taking this into account, it is desirable that the total pressure ofgas be within a range of greater than 2.7 Pa and at most 7.0 Pa.

Next, the dependence of driving voltage on the partial pressure of theoxygen gas is analyzed by referring to experiment results for hole-onlydevices 1B formed with the total pressure being set to 2.7 Pa and theinput power being set to 500 W (devices No. 13, 10, and 1). Whencomparing such hole-only devices 1B, it was observed that, at leastwhile the partial pressure of the oxygen gas is within the range of atleast 50% and at most 70% of the total pressure of gas, the drivingvoltage decreases as the partial pressure of the oxygen gas increases,as illustrated in FIG. 3B. Meanwhile, the present inventors observedthrough another experiment that when the partial pressure of the oxygengas exceeds this range, the driving voltage increases adversely. Takingthis into account, it is desirable that the oxygen gas partial pressurebe set to at least 50%, while setting an upper limit at approximately70%.

Finally, the dependence of driving voltage on input power is analyzed byreferring to experiment results for hole-only devices 1B formed with thetotal pressure being set to 4.8 Pa and the partial pressure of theoxygen gas being set to 50% (devices No. 14, 2, and 8). When comparingsuch hole-only devices 1B, the driving voltage rapidly increases whenthe input power exceeds 500 W, as illustrated in FIG. 3C. Taking thisinto account, it is desirable that input power be set to at most 500 W.Here, it should be noted that, the experiment results for devices No. 1and 3 in Table 3 indicate that even when the input power is set to 500W, driving voltage increases when the total pressure is no greater than2.7 Pa.

FIG. 4 illustrates current density-applied voltage curves for thehole-only devices 1B. In FIG. 4, curves corresponding to devices No. 14,1, and 7 are illustrated for example. In FIG. 4, the vertical axisindicates current density (mA/cm²), whereas the horizontal axisindicates applied voltage (V). The hole-only device No. 14 is ahole-only device formed with the desirable conditions being fulfilledfor each film forming condition, i.e., the total pressure of gas, thepartial pressure of the oxygen gas, and input power. On the other hand,devices No. 1 and 7 are hole-only device formed with the desirableconditions not being fulfilled for at least one film forming condition.

In order to facilitate explanation, hereinafter, the film formingconditions under which the hole injection layer 4 in device No. 14 wasformed is referred to as film forming conditions A, the film formingconditions under which the hole injection layer 4 in device No. 1 wasformed is referred to as film forming conditions B, and the film formingconditions under which the hole injection layer 4 in device No. 7 wasformed is referred to as film forming conditions C. In accordance,devices No. 14, 1, and 4 are respectively labeled HOD-A, HOD-B, andHOD-C in FIG. 4.

As illustrated in FIG. 4, the current density-applied voltage curve forHOD-A indicates a rise at a lower applied voltage compared to those forHOD-B and HOD-C, and reaches a high current density at a lower appliedvoltage compared to those for HOD-B and HOD-C. Based on this, assumptionis made that the hole injection efficiency from the hole injection layer4 to the buffer layer 6A in the HOD-A is higher than that in HOD-B andHOD-C. Further, HOD-A operates at the lowest driving voltage among thehole-only devices 1B.

In the above, observation has been made of the hole injection efficiencyfrom the hole injection layer 4 to the buffer layer 6A in the hole-onlydevices 1B. Here, as already described above, the structure of thehole-only devices 1B and the structure of the organic EL element 1 aresimilar, differing only in the cathodes. Therefore, the hole injectionefficiency from the hole injection layer 4 to the buffer layer 6A in theorganic EL element 1 is dependent upon the film forming conditions, inthe same way as the hole injection efficiency from the hole injectionlayer 4 to the buffer layer 6A in the hole-only devices 1B is dependentupon the film forming conditions. In order to confirm this, the presentinventors prepared three organic EL elements 1 having hole injectionlayers 4 formed under the respective film forming conditions A, B, andC.

More specifically, as illustrated in FIG. 1, each of the organic ELelements 1 prepared by the present inventors for this experimentincludes a substrate 10 and a 50 nm-thick anode 2 made of an ITO thinfilm and formed on the substrate 10, and further includes the followinglayers disposed above the anode 2 in the stated order: a 30 nm-thickhole injection layer 4 made of tungsten oxide; a 20 nm-thick bufferlayer 6A made of TFB, which is an organic amine-containing organic highmolecular material; a 70 nm-thick light-emitting layer 6B made of F8BT,which is an organic high molecular material; and a cathode 8C composedof a 5 nm-thick barium layer and a 100 nm-thick aluminum layer. Notethat, considering that the organic EL elements 1 here are assessmentdevices, the banks 5 were not included therein.

Each of the organic EL devices 1 corresponding to the respective filmforming conditions A, B, and C so prepared were then connected to thedirect current power supply DC, and voltage was applied thereto. FIG. 5illustrates current density-applied voltage curves for the organic ELdevices 1 corresponding to the respective film forming conditions A, B,and C. In FIG. 5, the vertical axis represents current density (mA/cm²),and the horizontal axis represents applied voltage (V).

In order to facilitate explanation, hereinafter, the organic El elements1 corresponding to the film forming conditions A, B, and C arerespectively referred to as BPD-A, BPD-B, and BPD-C. This is inaccordance with the illustration provided in FIG. 5.

As illustrated in FIG. 5, the current density-applied voltage curve forBPD-A indicates a rise at a lower applied voltage compared to those forBPD-B and BPD-C, and reaches a high current density at a lower appliedvoltage compared to those for BPD-B and BPD-C. This trend is similar tothe trend which could be seen in the hole-only devices HOD-A, HOD-B, andHOD-C, which were prepared under the same respective film formingconditions as BPD-A, BPD-B, and BPD-C.

FIG. 6 illustrates light-emission intensity-current density curves forthe above organic EL elements 1. Each curve in FIG. 6 indicateslight-emission intensities for different current densities. In FIG. 6,the vertical axis indicates light-emission intensity (cd/A), whereas thehorizontal axis indicates current density (mA/cm²). According to FIG. 6,it can be seen that, at least within the range of current densitiesmeasured in the experiment, BPD-A has the highest light-emissionintensity among the three organic EL elements 1.

From the above results, assumption is made that the hole injectionefficiency from the hole injection layer 4 to the buffer layer 6A in theorganic EL element 1 is dependent upon the film forming conditions, inthe same way as the hole injection efficiency from the hole injectionlayer 4 to the buffer layer 6A in the hole-only devices 1B is dependentupon the film forming conditions. That is, assumption is made thatexcellent hole injection efficiency from the hole injection layer 4 tothe buffer layer 6A is realized, and accordingly, excellent low voltagedrive and high light-emission efficiency of the organic EL element 1 arerealized by forming the hole injection layer 4 by performing reactivesputtering in a DC magnetron sputtering device, by using metal tungstenas the sputtering target, not performing any control of substratetemperature, and using a chamber gas composed of argon gas and oxygengas, and further, by performing the reactive sputtering under thepredetermined film forming conditions such that: the pressure of the gasintroduced into the sputtering device (the total pressure of gas) isgreater than 2.7 Pa and at most 7.0 Pa; the partial pressure of theoxygen gas is at least 50% and at most 70% of the total pressure of gas;and the input power density per unit area of the sputtering target is atleast 1 W/cm² and smaller than 2.8 W/cm².

Note that in the above, the film forming condition concerning inputpower is presented by using input power density instead, in accordancewith Table 2. Accordingly, when using a DC magnetron sputtering devicethat is different from the DC magnetron sputtering device used in thepresent experiment, a hole injection layer 4 that realizes excellent lowvoltage drive and high light-emission efficiency of the organic ELelement 1 as that in the present embodiment is yielded by adjustinginput power according to the size of the sputtering target so that inputpower density fulfills the condition concerning input power in theabove-described film forming conditions. Further, note that in theabove-described film forming conditions, the condition concerning thetotal pressure of gas and the condition concerning the partial pressureof oxygen gas remain the same regardless of such factors as the deviceused for the film forming and the size of the sputtering target.

Additionally, when forming the hole injection layer 4 by reactivesputtering in the sputtering device, no deliberate adjustment ofsubstrate temperature is performed in the sputtering device, which isassumed to be placed under room temperature. Therefore, the substrate isat room temperature at least before the forming of the hole injectionlayer 4. However, it should be noted that there is a possibility of thesubstrate temperature increasing by several tens of degrees Celsiusduring the forming of the hole injection layer 4.

Furthermore, the organic EL element 1 having the hole injection layer 4formed under film forming conditions A corresponds to the organic ELelement 1 in embodiment 1, having the occupied energy level near theFermi surface. Further observation is to be made regarding this point inthe following.

(Electronic State of Hole Injection Layer)

The tungsten oxide composing the hole injection layer 4 in the organicEL element 1 pertaining to embodiment 1 has the occupied energy levelnear the Fermi surface. The occupied energy level near the Fermi surfaceis formed by adjusting the film forming conditions under which the holeinjection layer 4 is formed in the manner indicated through the aboveexperiments. Details concerning this point are provided in thefollowing.

The present inventors conducted an experiment to check whether or notthe occupied energy level near the Fermi surface exists in tungstenoxide films formed under the respective film forming conditions A, B,and C.

The present inventors prepared sample devices for photoelectronspectroscopy measurement by applying the respective film formingconditions A, B, and C. More specifically, each of the sample devices inthe experiment has a structure indicated by 1A in FIG. 7, where a 10nm-thick tungsten oxide layer 12 (corresponding to the hole injectionlayer 4) is formed on a conductive silicon substrate 11 by reactivesputtering. In the following, a sample device 1A formed under filmforming conditions A is referred to as sample device A, a sample device1A formed under film forming conditions B is referred to as sampledevice B, and a sample device 1A formed under film forming conditions Cis referred to as sample device C.

After forming the tungsten oxide layers 12 inside a sputtering device,the sample devices A, B, and C were then transported to a gloveboxconnected to the sputtering device and filled with nitrogen gas toprevent atmospheric exposure. Subsequently, the sample devices A, B, andC were sealed inside transfer vessels in the glovebox, and were thenmounted on a photoelectron spectroscopy device. Thus, ultravioletphotoelectron spectroscopy (UPS) measurement of the sample devices A, B,and C was performed while preventing the tungsten oxide layers 12 fromundergoing atmospheric exposure after forming thereof.

Commonly, a UPS spectrum obtained as a result of the UPS measurementreflects a state of occupied energy levels, such as a valence band,within several nanometers in distance from the surface of the target ofmeasurement. As such, the present experiment was conducted to observethe state of occupied energy levels at a surface portion of the tungstenoxide layer 12 by utilizing UPS measurement.

The conditions under which the UPS measurement was conducted are asfollows. Here, it should be noted that since a conductive siliconsubstrate (i.e., the conductive silicon substrate 11) is used in each ofthe sample devices A, B, and C, charge-up did not occur during the UPSmeasurement of the sample devices A, B, and C.

Light source: He I line

Bias: None

Electron emission angle: Direction of normal line to the substrate

Interval between measurement points: 0.05 eV

FIG. 8 illustrates a UPS spectrum of the tungsten oxide layer 12 of thesample device A. The origin of the horizontal axis, which representsbinding energy, corresponds to the Fermi surface of the conductivesilicon substrate 11, and the left direction with respect to the originis positive.

In the following, description is provided on the occupied energy levelsof the tungsten oxide layer 12 with reference to FIG. 8.

Typically, a UPS spectrum of tungsten oxide indicates a large and rapidrise which can be uniquely distinguished from other areas thereof. Here,a tangential line passing through an inflection point of the above riseis illustrated as line (i), and the point at which line (i) intersectsthe horizontal axis is illustrated as point (iii). Further, a UPSspectrum of tungsten oxide can be divided into two areas: area x thatextends in the high binding energy direction from point (iii), and areay that extends in the low binding energy direction from point (iii).

Here, according to the composition ratios of the tungsten oxide layers12 given in Table 4, the ratio between tungsten atoms and oxygen atomsis approximately 1:3 in each of the sample devices A, B, and C. Thecomposition ratios in Table 4 were obtained by performing X-rayphotoelectron spectroscopy (XPS) measurement with respect to thetungsten oxide layers 12. More specifically, XPS measurement of thesample devices was performed by using the photoelectron spectroscopydevice while preventing atmospheric exposure of the tungsten oxide layer12 in the sample devices, in a similar way as in the UPS measurement. Byconducting the XPS measurement, the composition ratio between tungstenand oxygen within several nanometers in distance from the surface of thetungsten oxide layer 12 was estimated for each of the sample devices A,B, and C. Note that the film forming conditions under which the tungstenoxide layer 12 in each of the sample devices A, B, and C was formed arealso illustrated in Table 4.

TABLE 4 Sample Sample Sample Sample Device Device A Device B Device CFilm Forming Conditions Film Forming Film Forming Film FormingConditions A Conditions B Conditions C Total Pressure (Pa) 4.8 2.7 2.7Oxygen Partial Pressure (%) 50 70 70 Input Power (W) 250 500 1000Composition Ratio 3.0 2.9 2.8 (Oxygen/Tungsten)

According to the composition ratios, assumption is made that in each ofthe samples A, B, and C, the tungsten oxide layer 12, at least withinseveral nanometers in distance from the surface thereof, has an atomicarrangement based on that of tungsten trioxide. That is, the tungstenoxide layer 12 in each of the samples A, B, and C has a basic structurewhere six oxygen atoms bond with one tungsten atom in octahedralcoordination and the octahedrons share oxygen atoms at corners thereofwith adjacent octahedrons. As such, the area x in FIG. 8 corresponds toan energy level deriving from the above-described basic structure, andis an area corresponding to a so-called valence band. Here, the occupiedenergy level deriving from the basic structure is an occupied energylevel possessed by a tungsten trioxide crystal, or by an amorphousstructure in which crystalline order is disturbed (yet in which bondsare not broken, thus the above basic structure being preserved). Notethat the present inventors performed X-ray absorption fine structure(XAFS) measurement with respect to the tungsten oxide layer 12 of eachof the sample devices A, B, and C, and have confirmed that theabove-mentioned basic structure is formed in each of the sample devicesA, B, and C.

Based on the above, the area y in FIG. 8 corresponds to the band gapbetween the valence band and the conduction band of the tungsten oxidelayer 12. Further, as is suggested by the UPS spectrum in FIG. 8, it isknown that an occupied energy level that differs from the occupiedenergy level of the valence band may exist in the area y of the tungstenoxide. This occupied energy level in area y derives from a structuredifferent from the above-mentioned basic structure, and is referred toas a band gap energy level (in-gap state or gap state).

FIG. 9 illustrates a UPS spectrum of the tungsten oxide layer 12 foreach of the sample devices A, B, and C within area y. The spectrumintensity indicated by the vertical axis in FIG. 9 has been normalizedusing the peak-top intensity value of a peak (ii) in FIG. 8, which islocated approximately between 3 eV and 4 eV in the high binding energydirection from point (iii). In addition, note that point (iii) in FIG. 9is illustrated at the same point on the horizontal axis as in FIG. 8. InFIG. 9, the horizontal axis indicates a relative value (relative bindingenergy) with respect to point (iii), and the binding energy decreasesfrom left to right.

As illustrated in FIG. 9, the spectrum indicating the tungsten oxidelayer 12 in the sample device A exhibits a peak within a binding energyrange from 3.6 eV lower than point (iii) to 1.8 eV lower than point(iii). The point at which this peak clearly begins is labeled as point(iv) in FIG. 9. Here, it should be noted that the existence of such apeak is not observed in the spectra corresponding to sample devices Band C.

As such, the present invention uses, as the hole injection layer, a filmof tungsten oxide whose UPS spectrum indicates an upward protrusion (notnecessarily a peak) within a binding energy range from 1.8 eV lower thanpoint (iii) to 3.6 eV lower than point (iii). Due to this, the organicEL element pertaining to the present invention exhibits excellent holeinjection efficiency.

Furthermore, it has been found that the hole injection efficiency in theorganic EL element tends to increase when the upward protrusion has ahigher degree of sharpness. Therefore, as illustrated in FIG. 9, it canbe said that the binding energy range from 2.0 eV lower than point (iii)to 3.2 eV lower than point (iii) is particularly important, since theupward protrusion is relatively easier to confirm and has a relativelysharp inclination within this binding energy range.

Note that, in the following, the above-described upward protrusionobserved in the UPS spectrum is referred to as “a spectral protrusionnear the Fermi surface”. Furthermore, the energy level corresponding tothis spectral protrusion near the Fermi surface is the “the occupiedenergy level near the Fermi surface”, which has been already describedin the above.

Subsequently, the present inventors performed differentiation on thenormalized intensity of the UPS spectrum of each of the sample devicesA, B, and C illustrated in FIG. 9. The differentiation was performed forthe purpose of making the spectral protrusion near the Fermi surfacemore distinguishable.

In specific, by using the graph analysis software IGOR Pro 6.0, thepresent inventors conducted binomial smoothing (where the smoothingfactor was set to 1) 11 times with respect to the UPS spectraillustrated in FIG. 9, and then performed differentiation by applyingthe central difference method. Such processing of data was conducted soas to remove fluctuation factors such as background noise generatedduring the UPS measurement, to smoothen the differential curves, and toconvey the arguments presented in the following in as clear a manner aspossible.

FIG. 10 illustrates differential curves yielded as a result of the aboveprocessing. Points (iii) and (iv) in FIG. 10 are provided at the samepoints on the horizontal axis as the respective points (iii) and (iv) inFIG. 9.

According to the differential curves illustrated in FIG. 10, thederivatives of normalized intensities of the tungsten oxide layers 12corresponding to sample devices B and C do not depart from the vicinityof “0” within an area (v). Area (v) indicates an area which extends froma point which indicates a minimal binding energy that can be measured byusing the photoelectron spectroscopy device to point (iv). Furthermore,in an area (vi) extending approximately 1.2 eV in the high bindingenergy direction from point (iv), the derivatives corresponding tosample devices B and C exhibit only a slight increase as approaching thehigh binding energy direction, although increase is observed in theincrease rates thereof. Here, the shapes of the differential curvescorresponding to sample devices B and C within areas (v) and (vi) turnout to exhibit similarity with the respective UPS spectra illustrated inFIG. 9, from which the differential curves of FIG. 10 have been derived.Therefore, it can be said that the shapes of the UPS spectrum and thedifferential curve derived therefrom of each of sample devices B and Cwithin areas (v) and (vi) resemble the shape of an exponential functioncurve.

On the other hand, the differential curve for the tungsten oxide layer12 of sample A exhibits a rapid rise from around point (iv) towards thedirection of higher binding energy. Thus, the shape of the differentialcurve within the areas (v) and (vi) clearly differs from the shape of anexponential function curve. Similarly, in FIG. 9, the spectrumcorresponding to the tungsten oxide layer 12 of the sample device A,from which the differential curve in FIG. 10 corresponding to sampledevice A is derived, begins to protrude in the vicinity of point (iv).At the same time, it could also be seen that the spectrum in FIG. 9exhibits a spectral protrusion near the Fermi surface, which is notfound in a spectrum having the shape of an exponential function curve.

In other words, sample A as described above is characterized in that anoccupied energy level near the Fermi surface is found within a bindingenergy range from approximately 1.8 eV to approximately 3.6 eV lowerthan the lowest binding energy of the valence band. In particular, thespectral protrusion near the Fermi surface, corresponding to theoccupied energy level near the Fermi surface, can be clearly seen in theUPS spectrum in a binding energy range from approximately 2.0 eV toapproximately 3.2 eV lower than the lowest binding energy of the valenceband.

Next, the present inventors conducted atmospheric exposure of thetungsten oxide layer 12 of each of the sample devices A, B, and C for aperiod of one hour at normal temperature. Note that the sample devicesA, B, and C are the same sample devices as used in conducting the UPSmeasurement to obtain the UPS spectra in FIG. 9, and therefore have notbeen exposed to the atmosphere since the forming of the tungsten oxidelayers 12 therein. Following atmospheric exposure, the present inventorsconducted UPS measurement once again with respect to the tungsten oxidelayer 12 in each of the sample devices A, B, and C. The measurement wasconducted focusing on the changes in the UPS spectrum corresponding toeach of the sample devices A, B, and C. FIG. 11 illustrates UPS spectrawithin the above area y. The horizontal axis in FIG. 11 is similar tothe horizontal axis in FIG. 9, and points (iii) and (iv) in FIG. 11 areprovided at the same points on the horizontal axis as the respectivepoints (iii) and (iv) in FIG. 9.

Based on the UPS spectra illustrated in FIG. 11, the tungsten oxidelayer 12 in each of the samples B and C does not exhibit the spectralprotrusion near the Fermi surface either before or after atmosphericexposure. Contrariwise, it can be seen that the tungsten oxide layer 12of the sample device A still exhibits the spectral protrusion near theFermi surface after atmospheric exposure, although the spectrum differsin intensity and shape compared to before atmospheric exposure. Hence,it is observed that the sample device A retains the same characteristicsbefore and after atmospheric exposure performed for a certain period oftime. Therefore, it can be concluded that the sample device A has acertain level of stability with respect to ambient atmosphere.

In the above, description has been provided focusing on the UPS spectraobtained by performing UPS measurement with respect to the sampledevices A, B, and C. However, it should be noted that the existence ofthe spectral protrusion near the Fermi surface is similarly observed inspectra obtained by performing XPS measurement and hard X-rayphotoemission spectroscopy measurement.

FIG. 12 illustrates an XPS spectrum of the tungsten oxide layer 12 inthe sample device A after the atmospheric exposure as described above.Note that, in FIG. 12, the XPS spectrum is overlaid with a UPS spectrumof the tungsten oxide layer 12 in the sample device A (the same UPSspectrum as illustrated in FIG. 8), so as to enable comparison betweenthe two spectra.

The conditions under which the XPS measurement was conducted are similarto the above-described conditions under which the UPS measurement wasconducted, differing only in that an Al—K alpha line was used as thelight source. Also, the interval between measurement points was set to0.1 eV, differing from the UPS measurement. In FIG. 12, point (iii) isprovided at the same point on the horizontal axis as point (iii) in FIG.8, and further, the horizontal axis indicates a relative binding energywith respect to point (iii), similar as in FIG. 9. In addition, a linein the XPS spectrum corresponding to the line (i) in FIG. 8 isillustrated as line (i)′ in FIG. 12.

As illustrated in FIG. 12, the spectral protrusion near the Fermisurface is found in the XPS spectrum of the tungsten oxide layer 12 inthe sample A, as well as in the UPS spectrum, as a protrusion of aconsiderable degree within a binding energy range from approximately 1.8eV to approximately 3.6 eV lower than the lowest binding energy of thevalence band. Further, the spectral protrusion near the Fermi surfacewas similarly found when the present inventors performed a hard X-rayphotoemission spectroscopy measurement with respect to the tungstenoxide layer 12 in the sample A.

In the above, the sample device 1A (illustrated in FIG. 7) was used,instead of the organic EL element 1 which is illustrated in FIG. 1, asthe sample device for conducting the photoelectron spectroscopymeasurements. The sample device 1A has a structure where the tungstenoxide layer 12 is formed on the conductive silicon substrate 11. Here,it should be noted that this measure has been taken merely for the sakeof preventing the occurrence of charge-up during measurement, and thus,the structure of the organic EL element pertaining to the presentinvention is not to be limited to the structure of the sample device 1A.

According to another experiment conducted by the present inventors, whenUPS measurement and XPS measurement were conducted by using sampledevices having the structure of the organic EL element 1 illustrated inFIG. 1 (i.e., the structure where the anode 2 made of ITO and the holeinjection layer 4 made of tungsten oxide are formed in the stated orderon one surface of the substrate 10), the occurrence of charge-up wasencountered during the measurement of tungsten oxide layers formed underfilm forming conditions B and C.

However, when a neutralizing electron gun for cancelling charge-up wasused in such measurements, although there were cases where the absolutevalues of the binding energy indicated by the occupied energy levels ofthe hole injection layer 4 (for example, the binding energy value whenthe Fermi surface of the photoelectron spectroscopy device itself isdetermined as the reference point) differed from that of the tungstenoxide layer 12 of the sample device 1A, a spectrum having a similarshape as the spectrum of the sample device 1A was obtained, at leastwithin a binding energy range extending from the band gap energy levelto the lowest binding energy of the valence band.

(Observation on Hole Injection Efficiency from Hole Injection Layer toFunctional Layer)

The principle of the effect that the existence of the occupied energylevel near the surface in the hole injection layer made of tungstenoxide, observed as a spectral protrusion near the Fermi surface in, forinstance, a UPS spectrum of the hole injection layer, has on holeinjection efficiency can be explained as provided in the following.

It has been frequently reported, with reference to results ofexperiments and first principles calculations, that the occupied energylevel near the Fermi surface, found in thin films or crystals oftungsten oxide, derives from an oxygen vacancy structure or a similarstructure.

More specifically, assumption has been made that the occupied energylevel near the Fermi surface derives from a binding orbital formed by 5dorbitals of adjacent tungsten atoms due to an oxygen atom vacancy orfrom 5d orbitals of single tungsten atoms which are not terminated withan oxygen atom and which exist at the surface or within a tungsten oxidefilm. Further, assumption has been made that such 5d orbitals, either insemi-occupied state or unoccupied state and when coming in contact withan organic molecule, are capable of pulling an electron off of the HOMOof the organic molecule for mutual stabilization of energetic states.

As a matter of fact, a report has been made that, when a layer composedof α-NPD, which is an organic low molecular material, is layered on athin film of molybdenum oxide, an electron transfers from the α-NPDmolecule to the molybdenum oxide thin film (Non-Patent Literature 2).Note that molybdenum oxide has many physical properties in common withtungsten oxide, such as a catalyst effect, electrochromism, andphotochromism.

Meanwhile, the present inventors have made an assumption that, in thecase of tungsten oxide, a semi-occupied 5d orbital of a single tungstenatom, which is lower in terms of binding energy than a binding orbitalof 5d orbitals of adjacent tungsten atoms, or a structure similar tosuch a semi-occupied 5d orbital corresponds to the occupied energy levelnear the Fermi surface.

FIG. 13 illustrates an energetic state at an interface between atungsten oxide layer that has the occupied energy level near the Fermisurface pertaining to the present invention, and an α-NPD layer.

First of all, FIG. 13 illustrates the lowest binding energy of thevalence band (the “upper end of the valence band” in FIG. 13) of thetungsten oxide layer (corresponding to the hole injection layer), andthe lowest binding energy of the occupied energy level near the Fermisurface of the tungsten oxide layer, corresponding to the point at whichthe spectral protrusion near the Fermi surface begins to rise (the“upper end of the in-gap state” in FIG. 13). In the UPS spectrum, theupper end of the valence band corresponds to point (iii) in FIG. 8, andthe upper end of the in-gap state corresponds to point (iv) in FIG. 9.

In addition, FIG. 13 illustrates the relation between the tungsten oxidelayer and the α-NPD layer (corresponding to a functional layer) when theα-NPD layer is layered on the tungsten oxide layer. Specifically, toillustrate such a relation, illustration is provided of a thickness ofthe α-NPD layer, a binding energy of the HOMO of the α-NPD, and a vacuumlevel of the α-NPD layer. Here, the binding energy of the HOMO of theα-NPD layer corresponds to a binding energy in a UPS spectrum of theα-NPD layer at a point at which a peak of binding energy correspondingto the HOMO begins. In other words, the binding energy of the HOMO isthe lowest binding energy of the HOMO of the α-NPD.

More specifically, the energy diagram illustrated in FIG. 13 is obtainedthrough repeated alternate execution of the UPS measurement and ultrahigh vacuum vapor deposition, where the tungsten oxide layer, which isformed on an ITO substrate, is transferred back and forth between aphotoelectron spectroscopy device and a ultra high vacuum vapordeposition device connected to the photoelectron spectroscopy device.Since the occurrence of charge-up was not encountered during the UPSmeasurement, the binding energy on the vertical axis in FIG. 13 isindicated as an absolute value taken with respect to a reference point,which is the Fermi level of the ITO substrate.

FIG. 13 shows that, at least within a range of 0 nm to 0.3 nm from thesurface of the α-NPD layer, i.e. in a vicinity of the interface betweenthe tungsten oxide layer and the α-NPD layer, the upper end of thein-gap state of the tungsten oxide layer and the HOMO of the α-NPD layerapproximately equal one another in terms of binding energy. In otherwords, the energy levels are in a state of alignment (the statedescribed above as the “interface energy level alignment”). Here, itshould be noted that the state of being “approximately equal” asreferred to above actually includes a state where a slight differenceexists between the binding energies of the two layers, and denotes arange of ±0.3 eV inclusive, to be specific.

Further, FIG. 13 shows that the interface energy level alignment isformed as a result of interaction between the tungsten oxide and theα-NPD, and not by mere coincidence.

For instance, the change in vacuum level (vacuum level shift) of theα-NPD observed at the interface between the tungsten oxide layer and theα-NPD layer indicates that an electrical double layer (EDL) is formed atthe interface. Considering the direction of the vacuum level shift, theEDL is formed to be negative in the direction of the tungsten oxidelayer and positive in the direction of the α-NPD layer. In addition,since the magnitude of the vacuum level shift is considerably largebeing near 2 eV, it is reasonably assumed that the EDL has been formednot due to physical adsorption or the like, but rather as a result of achemical bond or a similar effect. That is, there is enough evidence toassume that the interface energy level alignment as mentioned above hasbeen formed as a result of interaction between the tungsten oxide andthe α-NPD.

Specifically, the present inventors assume that the interaction iscaused by a mechanism as described in the following.

First of all, as described above, the occupied energy level near theFermi surface derives from the 5d orbital of a tungsten atom, whichforms an oxygen vacancy structure or a similar structure. In thefollowing, the 5d orbital of the tungsten atom is referred to as a “W5dorbital corresponding to the spectral protrusion”.

When the HOMO of the α-NPD molecule approaches the W5d orbitalcorresponding to the spectral protrusion at the surface of the tungstenoxide layer, an electron transfers from the HOMO of the α-NPD moleculeto the W5d orbital corresponding to the spectral protrusion for themutual stabilization of energetic states. Hence, an EDL is formed at theinterface, thus causing the vacuum level shift as seen in FIG. 13 andthe interface energy level alignment.

More specifically, many reports have been made, as a result of firstprinciples calculations, that the electron density of the HOMO of anorganic amine-containing molecule, such as α-NPD, typically exhibits adistribution biased towards a nitrogen atom in the amine structure, andthat the main component of the HOMO of an organic amine-containingmolecule is a lone pair of electrons of the nitrogen atom. Assumption istherefore made that, particularly, at the interface between the tungstenoxide layer and a layer of an organic amine-containing molecule, anelectron transfers from the lone pair of electrons of the nitrogen atomin the amine structure of the amine-containing molecule to the W5dorbital corresponding to the spectral protrusion.

This assumption is supported by reports made of an interface energylevel alignment, similar to the interface energy level alignment betweenthe tungsten oxide layer and the α-NPD layer illustrated in FIG. 13,being formed at an interface between a vapor deposition film ofmolybdenum oxide, which has a physical property similar to tungstenoxide as described above, and α-NPD or F8BT (refer to Non-PatentLiteratures 3, 4, and 5).

The excellent hole injection efficiency, with respect to the one or morefunctional layers, of the hole injection layer of the organic EL elementpertaining to the present invention is explained by the interface energylevel alignment as described above. That is, an interface energy levelalignment is formed between the hole injection layer made of tungstenoxide that has the occupied energy level near the Fermi surface, and theadjacent one of the one or more functional layers. As a result, thebinding energy at the point at which the occupied energy level near theFermi surface begins to rise and the binding energy at the point atwhich the HOMO of the adjacent one of the one or more functional layersbegins to rise approximately equal one another. Further, the injectionof holes takes place between such energy levels having the interfaceenergy level alignment formed therebetween. Therefore, according to thepresent invention, the hole injection barrier between the hole injectionlayer and the adjacent one of the one or more functional layers isreduced to a nearly-nonexistent level.

Meanwhile, it is quite unlikely, in reality, that tungsten oxide existsthat does not include at all an oxygen vacancy structure or a similarstructure, which forms the occupied energy level near the Fermi surface.That is, it can be reasonably assumed that, for example, even in suchtungsten oxide as used to prepare each of the above-mentioned sampledevices B and C, whose photoelectron spectroscopy spectrum does notexhibit the spectral protrusion near the Fermi surface, an oxygenvacancy structure or a similar structure is present, however minimalthey may be in terms of number.

In view of the above, explanation is provided hereinafter with referenceto FIGS. 14A and 14B, of why the hole-only element HOD-A and the organicEL element BPD-A, both having a hole injection layer 4 corresponding tothe tungsten oxide layer 12 of the sample device A, exhibit particularlyexcellent hole injection efficiency as observed through the aboveexperiments.

When disposing a functional layer on a tungsten oxide layer, for theinteraction between the HOMO of the organic molecule composing thefunctional layer and the occupied energy level near the Fermi surface ofthe tungsten oxide layer to be triggered, a portion of the organicmolecule where the electron density of the HOMO is high and an oxygenvacancy structure or a similar structure at the surface of the tungstenoxide layer must approach (i.e. contact) each other to a certaindistance at the interface between the layers. The above-describedportion of the organic molecule is, for instance, a nitrogen atom in theamine structure of an organic amine-containing molecule, and isillustrated as “injection site y” in FIG. 14A. Further, an oxygenvacancy structure or a similar structure on the surface of the tungstenoxide layer is illustrated as “injection site x” in FIG. 14A.

However, in a tungsten oxide layer as incorporated in the samples B andC, the number density of injection sites x, if any, is extremely small,as illustrated in FIG. 14B, and it is for this reason that the upwardprotrusion near the Fermi surface does not appear in the UPS spectrumthereof. Thus, the possibility of the injection sites y and theinjection sites x coming into contact is extremely low. Since theinjection of holes takes place where the injection sites x and theinjection sites y come into contact, it is clear that hole injectionefficiency in samples B and C is extremely low.

In contrast to this, an abundance of injection sites y exists in atungsten oxide layer that exhibits the spectral protrusion near theFermi surface such as the tungsten oxide layer incorporated in theabove-mentioned sample device A, as illustrated in FIG. 14A. Thus, thereis a high possibility of the injection sites y and the injection sites xcoming into contact, and thus it is clear that the hole injectionefficiency from the hole injection layer to the functional layer is highwhen using such a tungsten oxide layer.

To further substantiate the observations made thus far, the energydiagram at the interface with the α-NPD layer was also measured, as inFIG. 13, for the tungsten oxide layer formed under film formingconditions C, in which no protrusion near the Fermi surface was observedat all.

FIG. 15 shows the results. As described above, the upper end of thein-gap state of the tungsten oxide layer, which corresponds to thespectral protrusion near the Fermi surface, was not observed at all inthe tungsten oxide layer formed under film forming conditions C.Accordingly, as a candidate for another energy level used in holeinjection, FIG. 15 shows the point at which a structure ((z) in FIG. 8)different from the spectral protrusion rises (the “upper end of secondin-gap state”). In the UPS spectrum, this point is observed in thehigher binding energy direction than the location of the spectralprotrusion near the Fermi surface. FIG. 15 also shows the upper end ofthe valence band of the tungsten oxide layer.

However, differing completely from FIG. 13, the HOMO of the α-NPD inFIG. 15 approaches neither the upper end of the second in-gap state northe upper end of the valence band. In other words, the interface energylevel alignment is not formed at all. This means that the second in-gapstate nor the valence band hardly interacts with the HOMO of the α-NPD.Further, even if holes can be injected from the upper end of the secondin-gap state to the HOMO of the α-NPD, the injection barrier is 0.75 eVand is an extremely large value compared to the case in FIG. 13, wherethe injection barrier is nearly zero.

Assumption is made that this difference in injection barrier greatlyaffects the driving voltage and the light-emission efficiency of thehole-only devices 1B and the organic EL elements 1 corresponding to thedifferent film forming conditions. Specifically, the differences incharacteristics between the hole-only devices 1B of the different filmforming conditions A, B, and C and between the organic EL elements 1 ofthe different film forming conditions A, B, and C strongly suggest thatthe organic EL element pertaining to the present invention has excellenthole injection efficiency from the hole injection layer to thefunctional layer.

When summarizing the above, the excellent hole injection efficiency ofthe organic EL element pertaining to the present invention is explainedas follows.

First, in the organic EL element pertaining to the present invention,the hole injection layer made of tungsten oxide exhibits, in thephotoelectron spectroscopy spectrum thereof, the spectral protrusionnear the Fermi surface. The existence of the spectral protrusion nearthe Fermi surface indicates that a considerable number of oxygen vacancystructures or similar structures, as well as occupied energy levels nearthe Fermi surface deriving from such structures, exist at the surface ofthe hole injection layer.

The occupied energy level near the Fermi surface pulls an electron offfrom the organic molecule composing one of the one or more functionallayers adjacent to the hole injection layer. As a result, the occupiedenergy level near the Fermi surface forms the interface energy levelalignment with the HOMO of the organic molecule.

As such, if a considerable number of oxygen vacancy structures orsimilar structures are present at the surface of the hole injectionlayer, the possibility increases of the occupied energy level near theFermi surface and a portion of the organic molecule where the electrondensity of the HOMO is high coming into contact with each other. Thus,the hole injection layer pertaining to the present invention efficientlyforms the interface energy level alignment with the adjacent one of theone or more functional layers, and accordingly, excellent hole injectionefficiency from the hole injection layer to the adjacent one of the oneor more functional layers is achieved.

(Relation Between Film Thickness Reduction of Tungsten Oxide Layer andHole Injection Characteristics, Driving Voltage, Etc.)

In the organic EL element 1 pertaining to the present invention, thehole injection layer 4 is yielded by firing the tungsten oxide filmimmediately after forming the tungsten oxide film under thepredetermined film forming conditions. The firing process is performedto apply heat and thereby densify the tungsten oxide film. Due to this,the hole injection layer 4, at the same as retaining the hole injectioncharacteristics as described above even after the firing process, isprovided with increased dissolution resistance to the etching solution,the cleaning liquid, etc., used in the bank forming process.

FIG. 16 is a graph illustrating the relation between driving voltages(normalized driving voltages) and the dissolution resistance of tungstenoxide (WOx) films formed under different film forming rates. Here, thedissolution resistance indicates the dissolution resistance that thetungsten oxide film exhibits when the etching solution (TMAH solution)is dropped on the tungsten oxide film immediately after the formingthereof. Here, note that in this experiment, the firing of the tungstenoxide film was not performed, and thus, control is performed of the filmdensity of the tungsten oxide film solely by using different filmforming rates, namely a “low rate”, an “intermediate rate”, and a “highrate”. In specific, the three film forming rates correspond to therespective conditions provided in the following.

Low rate: Power density=1.4 W/cm²; Ar/O₂ ratio in film formingatmosphere=100:100

Intermediate rate: Power density=2.8 W/cm²; Ar/O₂ ratio in film formingatmosphere=100:43

High rate: Power density=5.6 W/cm²; Ar/O₂ ratio in film formingatmosphere=100:43

Further, for the sake of assessment, the necessary dissolutionresistance of the tungsten oxide film with respect to film thicknessreduction was set, for example, such that when the film thicknessreduction amount of the tungsten oxide film is no greater than half thefilm thickness of the tungsten oxide layer immediately after the formingthereof (14 nm), the tungsten oxide film has the necessary dissolutionresistance. More specifically, the tungsten oxide film is considered ashaving the necessary dissolution resistance when the film thicknessreduction amount is no greater than 7 nm, in this example. In addition,the necessary performance level of the tungsten oxide film in terms ofdriving voltage (normalized driving voltage) was set, for example, suchthat the tungsten oxide layer has the necessary performance level whenthe driving voltage thereof is no greater than 1.

As shown by the graph in FIG. 16, as the film forming rate becomes ahigher rate, the film thickness reduction amount of the tungsten oxidefilm in the bank forming process decreases due to the dissolutionresistance increasing, while the driving voltage increases.Contrariwise, as the film forming rate becomes a lower rate, the filmthickness reduction amount of the tungsten oxide film increases due tothe dissolution resistance decreasing, while the driving voltagedecreases (improves). Here, it should be noted that when forming thetungsten oxide film at the low rate, the film thickness reduction amountincreases and thus it becomes difficult to ensure that the resultanthole injection layer has uniform film thickness over the entirety of alight-emission area. As such, the low rate poses a risk of such problemsas light-emission unevenness, etc., of the hole injection layer.

As described above, there is a trade-off relation between the filmthickness reduction amount of the tungsten oxide film (indicated by lineconnecting rhombuses) and driving voltage (indicated by the lineconnecting squares), in terms of the film density of the tungsten oxidefilm. That is, it is observed that the film thickness reduction amountof the tungsten oxide film decreases as the film density of the tungstenoxide film increases.

Subsequently, the present inventors assessed the characteristics oftungsten oxide films formed under different film forming conditions andsubjected to the firing processing after the forming thereof.Specifically, the present inventors prepared a plurality of samples oforganic EL panels each having a plurality of organic EL elements formedtherein (samples No. 1 through No. 7). Here, the hole injection layer ineach of the samples was formed under film forming/firing conditions(conditions changed including: film forming atmosphere, power density,film forming device used, and performance/non-performance of the firingprocess after the forming of the tungsten oxide film) differing from thefilm forming/firing conditions under which a hole injection layer in adifferent one of the samples was formed. Having formed such samples, thepresent inventors measured the film thickness of the hole injectionlayer in each of the samples at two areas, namely at a center area ofthe sample panel and at a peripheral area of the sample panel. Further,the present inventors assessed the samples in terms of: (i) film densityof the tungsten oxide film; (ii) film thickness reduction amount of thetungsten oxide film; (iii) device characteristics (driving voltage);(iv) dissolution resistance of the tungsten oxide film (resistanceagainst the etching solution and the cleaning liquid used in the bankforming process, also referred to as BNK resistance in the following);and (v) total performance (overall assessment including devicecharacteristics and dissolution resistance). The measurement of thedissolution resistance of the tungsten oxide film was performed byperforming the resist forming/removing process described in thefollowing in place of the bank forming process as a similar process.Specifically, the resist forming/removing process was performed by,after forming the tungsten oxide film in each of the samples, applyingresist (TFR-940 produced by Tokyo Ohka Kogyo Co., LTD.) by spin-coatingunder a condition of 2500 rpm/25 sec. Following the application ofresist, baking was performed at a temperature of 100° C. and for a 90second period, followed by developing by using a developer (TMAHsolution) having a concentration of 2.38%. Further, cleaning wasperformed for a 60 second period by using water. The resist so formedwas removed by using acetone.

Specifically, the assessment of dissolution resistance was performedsuch that when the film thickness reduction amount of a tungsten oxidefilm in the resist forming/removing process was no greater than half thefilm thickness of the tungsten oxide film immediately after the formingthereof, the tungsten oxide film was assessed as being desirable,whereas when the film thickness reduction amount was greater than halfthe film thickness of the tungsten oxide film immediately after theforming thereof, the tungsten oxide film was assessed as beingundesirable.

The assessment of device characteristics was performed such that whenthe driving voltage of a sample indicated a predetermined voltage valuesufficient to realize performance at least equivalent to that of an LCD,the sample was assessed as being desirable, whereas when the drivingvoltage of a sample indicated a greater value than the predeterminedvoltage value, the sample was assessed as being undesirable.

Table 5 shows the film forming/firing conditions, the measurementresults, and the assessment results. Note that here, samples 4 and 5 arethose samples achieved through implementation of the film forming/firingconditions in embodiment 1.

TABLE 5 WOx Film Forming Conditions and Film Thickness Reduction AmountAr:O₂ 100:100 100:100 100:100 100:100 100:100 100:43 100:43 PowerDensity 1.4 1.4 1.4 1.4 1.4 2.8 5.6 (W/cm²) WOx Firing Process NotPerformed Not Not Performed Performed Not Not Performed PerformedPerformed Performed Film Forming Large-sized Large-sized Large-sizedLarge-sized Large-sized Small-sized Small-sized Device Panel SurfaceCenter Center Edge Center Edge — — Measurement Position Film Density5.43 5.65 5.73 5.9 6.0 6.09 6.33 (g/cm³) Film Thickness 15 9.8 8 4.6 4 42.9 Reduction Amount (nm) Device Satisfactory — Satisfactory —Satisfactory Unsatisfactory Poor Characteristics BNK Resistance PoorPoor Poor Satisfactory Satisfactory Satisfactory Satisfactory TotalPerformance Poor Poor Poor Excellent Excellent Poor Poor Notes Resultsupon Data after Stable Operation of Device Initial Introduction ofDevice Temperature: Room Temperature/Resist: tok(TFR-940)/Spin-coating:2500 rpm for 25 sec/Baking: 100° C. for 90 sec Conditions: DevelopingSolution 2.38%/Developing for 60 sec + Washing with Water for 60sec/Resist Removal: Acetone

The results in Table 5 show that samples No. 6 an No. 7, for which thepower density was set relatively high and thus the film forming rate wasset to a higher rate, have high film density and thus desirable BNKresistance, but undesirable device characteristics. Accordingly, samplesNo. 6 and No. 7 were assessed as having undesirable total performance.Here, note that when forming samples No. 6 and No. 7, the firing processwas not performed since it was confirmed that the tungsten oxide filmstherein already had high dissolution resistance immediately after theforming thereof. As described above, samples No. 6 and No. 7 hadundesirable device characteristics. The present inventors conductedanother consideration and made an assumption that when tungsten oxidefilms are formed with the film forming rate set to a higher rate, itbecomes less likely that tungsten atoms with a valence of five areformed in the tungsten oxide, which results in the tungsten oxide filmshaving almost no oxygen vacancy structures generated therein (i.e., alltungsten atoms in the tungsten oxide films are tungsten atoms with avalence of six) and thus the device characteristics of the samplesincluding such tungsten oxide films being relatively low.

On the other hand, in each of samples No. 1 through No. 3, for which thepower density was set relatively low and the firing process was notperformed, a relatively large number of tungsten atoms having a valenceof five were generated in the tungsten oxide film due to film formingbeing performed at the low rate, and thus, oxygen vacancy structureswere formed in the film. Due to this, such samples have desirable holeinjection characteristics to some extent. However, the film thicknessreduction amount in each of such samples is relatively high due to thetungsten oxide film having undesirable BNK resistance. As such, adecrease in total performance is observed in such samples.

In contrast to the samples described above, in each of samples No. 4 andNo. 5, for which the power density was set relatively low (i.e., thefilm forming rate was set to the low rate) and the firing process wasperformed, tungsten atoms of the valence of five, and thus oxygenvacancy structures were formed in the tungsten oxide film. Thus, suchsamples exhibit excellent device characteristics, and at the same timehave excellent BNK resistance due to the film density of the tungstenoxide layer therein being improved. Hence, samples No. 4 and No. 5 wereassessed as having excellent total performance. The above resultsconfirm the effectiveness of the predetermined film forming conditionsand the firing process described in the examples of implementationdescribed in embodiment 1.

FIG. 17 is a graph illustrating the relation between film thicknessreduction amounts (indicated by the line connecting rhombuses) anddriving voltages (indicated by the line connecting circular dots) fordifferent film thicknesses of tungsten oxide (WOx) films. Here, notethat in this experiment, the firing of the tungsten oxide film was notperformed, and thus, control is performed of the film density of thetungsten oxide film solely by using different film forming rates.

As illustrated in FIG. 17, the driving voltage of the tungsten oxidefilm rapidly increases when the film density exceeds approximately 6g/cm³. This is assumed to be since the oxygen vacancy structures in thetungsten oxide film disappear and thus the occupied energy level nearthe Fermi surface in the tungsten oxide film also disappears, due to theincrease in film density. Meanwhile, the film density needs to beapproximately at least 5.8 g/cm³ in order to limit the film thicknessreduction amount to no greater than 7 nm. Further, in the graphillustrated in FIG. 17, there is a possibility of an inflection pointexisting near the film density of 5.8 g/cm3, which corresponds to apoint in the graph at which a line connecting the measurement pointscorresponding to film thickness reduction amounts lower than or equal toapproximately 5.7 g/cm³ and a line connecting the measurement pointscorresponding to film thickness reduction amounts higher than or equalto 5.9 g/cm³ intersect each other. Taking the above into consideration,assumption is made that as the film density of the tungsten oxide film,a film density within the range of at least 5.8 g/cm³ and at most 6.0g/cm³ is appropriate.

FIG. 18 is a diagram illustrating a UPS spectrum of a tungsten oxidefilm formed at the low rate, a UPS spectrum of a tungsten oxide filmformed at the low rate and later subjected to the firing processing(i.e., corresponding to the hole injection layer 4), and a UPS spectrumof a tungsten oxide film formed at the high rate, overlaid one on top ofanother.

As illustrated in FIG. 18, the UPS spectrum of the tungsten oxide filmformed at the high rate does not have the spectral protrusion near theFermi surface, which indicates that the occupied energy level near theFermi surface rarely exists in the tungsten oxide film.

On the other hand, the UPS spectrum of the tungsten oxide film formed atthe low rate has the spectral protrusion near the Fermi surface similaras in FIG. 9, which indicates that the occupied energy level near thesurface exists in the tungsten oxide film.

Further, when turning to the UPS spectrum of the tungsten oxide filmformed at the low rate and later subjected to the firing processing, thespectrum indicates that the occupied energy level near the Fermi surfaceexists in the tungsten oxide film.

As such, it is confirmed that the hole injection layer 4, even when thefiring process is performed, retains the occupied energy level near theFermi surface at an excellent state.

FIG. 19 illustrates the relation between film densities and the filmthickness reduction amounts for different tungsten oxide films. In thismeasurement, organic EL panels each having a plurality of organic ELelements formed therein were used. Further, in the graph in FIG. 19,“center” indicates an organic EL element near the center area of thepanel, and “edge” indicates an organic EL element near a peripheral areaof the panel.

It is confirmed from the results shown in FIG. 19 that the film densityof the tungsten oxide film increases when the firing process isperformed compared to when the firing processing is not performed, andas the film density increases, the film thickness reduction amountdecreases at both the center area of the panel and the peripheral areaof the panel.

In the following, description is provided on how the film thicknessreduction of the tungsten oxide film takes place and on the effects ofthe firing process performed after the forming of the tungsten oxidefilm, with reference to FIG. 20. When the tungsten oxide film is formedat the low rate and then the bank forming process is performed with thetungsten oxide film in its original state (at the same state as wheninitially formed), the film thickness reduction amount of the tungstenoxide film in the bank forming process reaches a considerable levelsince the tungsten oxide film in this case has low film density.Meanwhile, when the predetermined firing process is performed withrespect to the tungsten oxide layer having been formed, the film densityof the tungsten oxide film increases as a result of the application ofheat and the densification brought about in the firing process. Byperforming the firing process, the tungsten oxide film is provided withincreased dissolution resistance, and thus, even when the bank formingprocess is later performed, the film thickness reduction amount of thetungsten oxide film in the bank forming process is suppressed to as lowa level as possible.

(Surface Film Thickness Unevenness in Organic EL Panel)

FIG. 21 illustrates the relation between film thickness reductionamounts and surface film thickness unevenness in an organic EL panel,for different film densities of tungsten oxide films.

In the experiment described in the following, the present inventorsprepared a plurality of samples each including tungsten oxide filmsformed at a plurality of areas on the surface of a substrate. Further,the present inventors calculated the film thickness reduction amountwhen performing the firing process and the film thickness reductionamount when not performing the firing process, for different filmdensities of the tungsten oxide film. In addition, the present inventorscalculated the unevenness in film thickness between tungsten oxide filmson the substrate for different film densities of the tungsten oxidefilm. FIG. 21 shows the results. Here, note that the difference in filmthickness reduction amount between tungsten oxide films at differentareas on an organic EL panel, which is calculated as a differencebetween the maximum film thickness reduction amount and the minimum filmthickness reduction amount, is referred to as “panel surface filmthickness unevenness”.

Nowadays, one specification requirement that an organic EL panel needsto satisfy is that the panel surface film thickness unevenness betweenhole injection layers in the organic EL panel is no greater than 4 nm(i.e., to be within a range of ±2 nm, inclusive). This is since a greatpanel surface film thickness unevenness between hole injection layers inan organic EL panel may negatively affect the cavity design, etc., ofthe organic EL panel.

At present, it is difficult to completely prevent film thicknessreduction of tungsten oxide films from occurring. However, if a tungstenoxide film undergoing only relatively small film thickness reduction inthe bank forming process is realized, the initial film thickness of thetungsten oxide film, at the point when the forming thereof is completed,can be set to a relatively small thickness. This is advantageous in thatthe absolute amount of panel surface film thickness unevenness at thepoint when the manufacturing of the organic EL panel is completed can bereduced accordingly. In view of this, the present invention utilizes thetechnology of suppressing the film thickness reduction amount of thetungsten oxide film during the bank forming process to as low a level aspossible by providing the tungsten oxide film with improved dissolutionresistance. Due to this, the initial film thickness of the tungstenoxide film, at the point when the forming thereof is completed, can bereduced, and further, the absolute amount of panel surface filmthickness unevenness between hole injection layers in the organic ELpanel can be suppressed. Additionally, such technology also has anadvantageous effect of preventing ununiformity of light-emissionefficiency between organic EL elements formed in the panel.

Specifically, when turning to the results in FIG. 21, in each of thesamples used in this experiment, the panel surface film thicknessunevenness was suppressed to no greater than 4 nm (i.e., to be within arange of ±2 nm, inclusive), regardless of whether or not the firingprocess was performed. In particular, the panel surface film thicknessunevenness in the sample for which the firing process was performed waslower by at least 40% compared to the sample for which the firingprocess was not performed.

For achieving uniform light-emission characteristics in an organic ELpanel, it is desirable that the panel surface film thickness unevennessbe as small as possible. As such, the firing process performed withrespect to the tungsten oxide film in the present invention is effectivefor effectively suppressing the absolute amount of panel surface filmthickness unevenness.

FIG. 22 is a graph illustrating the relation between panel surface filmthickness unevenness (referred to hereinafter also as WOx film thicknessunevenness) between hole injection layers made of tungsten oxide andunevenness in current efficiency between organic EL elements of therespective colors R, G, and B. Here, measurement was performed of thelight-emission characteristics of organic EL elements each including acolor filter of one of the colors R, G, B layered therein.

As shown in FIG. 22, the unevenness in current efficiency betweenorganic EL elements increases in proportion with the increase in thepanel surface film thickness unevenness between tungsten oxide films inthe organic EL elements. Here, the present invention suppresses theunevenness in film thickness of hole injection layers between organic ELelements included in an organic EL panel, and thus realizes uniformcurrent efficiency and uniform light-emission characteristics of theorganic EL elements. This advantageous effect ultimately contributes toenhancing the image display performance of the entire organic EL panel.

Subsequently, the present inventors prepared organic EL panels eachhaving a plurality of organic EL elements disposed therein. Here, eachof the organic EL elements has, as a hole injection layer, a tungstenoxide film formed under the predetermined film-forming conditionsdefined in embodiment 1. Further, the organic EL elements in each of theorganic EL panels has disposed therein a color filter of one of thecolors R, G, and B. The present inventors then caused the organic ELpanels to drive and calculated the unevenness in current efficiencybetween the organic EL elements at the panel surface (maximum currentefficiency−minimum current efficiency). Then, the present inventorscompared the calculation results with those for example organic ELpanels for comparison (each having organic EL elements including, ashole injection layers, tungsten oxide films not having been subjected tothe firing process after the forming thereof and corresponding to one ofthe colors R, G, and B). Table 6 shows the results.

TABLE 6 Measurement Results of Unevenness in Panel Surface Efficiency(w/Color Filter) Color Filter (Maximum Value − Minimum Value)(%) ColorFiring Process Not Performed Firing Process Performed R ±2.0 ±1.3 G ±1.5±0.7 B ±2.3 ±1.4

As shown in Table 6, for each of the organic EL panel corresponding tothe color R, the organic EL panel corresponding to the color G, and theorganic EL panel corresponding to the color B, by performing the firingprocess with respect to the tungsten oxide films included therein afterthe forming of the tungsten oxide films, the unevenness in currentefficiency at the panel surface was reduced compared to when the firingprocess was not performed with respect to the tungsten oxide films.Thus, it is confirmed that each of the organic EL panels prepared can beexpected to exhibit uniform light-emission characteristics at all areasof the panel.

FIG. 23 is a graph illustrating the results of a measurement performedconcerning the relation between the film thickness reduction amount ofthe tungsten oxide film (indicated by the line connecting rhombuses) andthe film density of the tungsten oxide film (indicated by the lineconnecting squares) for different processing times of the firing processperformed after the forming of the tungsten oxide film. In thismeasurement, the thickness of the tungsten oxide film immediately afterthe forming thereof was set to 14 nm, and further, the firingtemperature in the firing process was set to 230° C.

As shown by the results in FIG. 23, the film thickness reduction amountof the tungsten oxide film becomes substantially constant when theprocessing time is approximately 15 minutes or longer. Further, the filmdensity of the tungsten oxide film becomes substantially constant whenthe processing time is approximately 45 minutes or longer. Meanwhile,even when the processing time was set to a longer time period thandescribed above, no prominent change in the characteristics of thetungsten oxide film was observed. According to such measurement results,in order to increase the film density of the tungsten oxide film andreduce the film thickness reduction amount of the tungsten oxide film inthe bank forming process, it suffices that the processing time of thefiring process be set to at least 15 minutes and at most 45 minutes. Assuch, assumption is made that the desirable processing time for thefiring process is within the range of at least 15 minutes and at most 45minutes.

Embodiment 2

<Overall Structure of Organic EL Element 1C>

FIG. 24A is a schematic cross-sectional view illustrating the structureof an organic EL element 1C according to the present embodiment. FIG.24B is a partially expanded view near a hole injection layer 4C.

The organic EL element 1C is, for example, an application type organicEL element including one or more functional layers each having beenformed by applying material in wet processing. The hole injection layer4A and a set of one or more functional layers, each containing organicmaterial and having a predetermined function, are disposed one on top ofthe other, and are disposed between a pair of electrodes, composed of ananode 2 and a cathode 8D.

Specifically, the organic EL element 1C includes a substrate 10 havingthe following layered on one main surface thereof in the stated order:the anode 2, an ITO layer 3, the hole injection layer 4A, a buffer layer6A, a light-emitting layer 6B, an electron injection layer 7, a cathode8D, and a sealing layer 9. The following description focuses on thedifferences of the organic El element 1C from the organic EL element 1.

(ITO Layer 3)

The indium tin oxide (ITO) layer 3 is provided between the anode 2 andthe hole injection layer 4A, and has the function of enhancing thebonding between the layers. In the organic EL element 1C, the ITO layer3 and the anode 2 are separate, but the ITO layer 3 may be consideredpart of the anode 2.

(Hole Injection Layer 4A)

Like the hole injection layer 4 in embodiment 1, the hole injectionlayer 4C is a layer of tungsten oxide formed under the predetermined“low rate” film forming conditions and having a film thickness of atleast 2 nm (30 nm in this example). Due to this, a Schottky ohmiccontact is formed between the ITO layer 3 and the hole injection layer4A, and thus, the difference in binding energy between the Fermi levelof the ITO layer 3 and the lowest binding energy of the occupied energylevel near the Fermi surface, at a position 2 nm away from the surfaceof the ITO layer 3 towards the hole injection layer 4A, is within therange of +0.3 eV inclusive. This results in the hole injection barrierbetween the ITO layer 3 and the hole injection layer 4A being moderatedcompared to the injection barrier between such layers in a conventionalstructure, and thus, the organic EL element 1C is able to operateexcellently at low voltage. In addition, the film density of the holeinjection layer 4A is set to a high density within a range of 5.8 g/cm³to 6.0 g/cm³, and thus, the tungsten oxide film that becomes the holeinjection layer 4A has increased dissolution resistance to the etchingsolution, the cleaning liquid, etc., that are used in the bank formingprocess for forming the banks 5. This results in the film thicknessreduction amount of the tungsten oxide film during the bank formingprocess being suppressed to as small an amount as possible. Note that asillustrated in FIG. 24A, the hole injection layer 4A has a concaveportion that is concave in the direction of the anode 2, at one surfacethereof on the side of the light-emitting layer 6B. The concave portionis formed due to a slight film thickness reduction occurring at thesurface of the hole injection layer 4A on the side of the light-emittinglayer 6B during the bank forming process.

In the composition formula WOx denoting the composition of the tungstenoxide constituting the hole injection layer 4A, x is a real numberexisting within a range of approximately 2<x<3. While it is desirablefor the hole injection layer 4A to be made of as pure a tungsten oxideas possible, the inclusion of a slight degree of impurities isacceptable, provided that the amount does not exceed the amount ofimpurities that might normally be incorporated.

Details on the predetermined film forming conditions for forming thehole injection layer 4A are provided in the sections “Method forManufacturing Organic EL Element 1C” and “Film forming Conditions forHole Injection Layer 4A”.

In embodiment 2, since the tungsten oxide layer constituting the holeinjection layer 4A is formed under predetermined film formingconditions, the hole injection layer 4A includes a plurality of tungstenoxide crystals 13, as illustrated in FIG. 24B. The particle diameter ofeach crystal 13 is on the order of nanometers. As an example, whereasthe thickness of the hole injection layer 4A is approximately 30 nm, theparticle diameter of the crystals 13 is approximately between 3 and 10nm. Hereinafter, the crystals 13, whose particle diameter is on theorder of nanometers, are referred to as “nanocrystals 13”, and a layeredstructure composed of the nanocrystals 13 is referred to as a“nanocrystal structure”. Note that apart from the nanocrystal structure,the hole injection layer 4A may include an amorphous structure.

In the hole injection layer 4A with the above nanocrystal structure, thetungsten atoms constituting the tungsten oxide are distributed toinclude both atoms at the maximum valence and atoms at a valence lessthan the maximum valence. Meanwhile, a tungsten oxide layer maytypically include the oxygen vacancy structure or a similar structure.Here, it should be noted that a tungsten atom not included in the oxygenvacancy structure or a similar structure has a valence of six, while atungsten atom included in the oxygen vacancy structure or a similarstructure has a valence less than the maximum valence of six.Furthermore, the oxygen vacancy structures or similar structures aretypically abundant at the surfaces of tungsten oxide crystals.

Accordingly, in the organic EL element 1C, in addition to the holeinjection barrier between the ITO layer 3 and the hole injection layer4A being moderated, a further improvement in hole conduction efficiencyis expected to be brought about by distributing tungsten atoms with avalence of five throughout the hole injection layer 4A to create theoxygen vacancy structures or similar structures. In other words, byproviding the hole injection layer 4A formed from tungsten oxide withthe nanocrystal structure, the holes injected from the ITO layer 3 tothe hole injection layer 4A are conducted by the oxygen vacancystructures or similar structures existing along the crystal interface ofthe nanocrystals 13. This increases the paths for conduction of holesand improves the hole conduction efficiency. This efficiently reducesthe driving voltage of the organic EL element 1C.

In addition to the above, the hole injection layer 4A is formed byperforming a predetermined firing process after the forming of thetungsten oxide film that later becomes the hole injection layer 4A. Dueto the firing process being performed, the film density of the tungstenoxide film is increased, which leads to the tungsten oxide film beingprovided with increased chemical resistance and dissolution resistance.Therefore, even if the tungsten oxide film comes into contact with adissolution solution, etc., used during processes performed afterforming of the tungsten oxide film, damage to the tungsten oxide filmand thus to the hole injection layer 4A due to dissolution,deterioration, or decomposition is reduced, and the film thicknessreduction amount of the tungsten oxide film in such processes performedafter the forming of the tungsten oxide film is effectively reduced.Further, due to the hole injection layer 4A being formed of materialhaving excellent chemical resistance, prevention of the decrease in thehole conduction efficiency of the hole injection layer 4A is alsoexpected.

Note that in the present embodiment, the hole injection layer 4A made oftungsten oxide may be formed of only the nanocrystal structure or may beformed of both the nanocrystal structure and the amorphous structures.Furthermore, it is desirable that the nanocrystal structure be presentthroughout the hole injection layer 4A. However, holes can beefficiently conducted from the lower edge of the hole injection layer 4Ato the upper edge of the hole injection layer 4A as long as at least oneconnection of grain boundaries extends from the interface where the ITOlayer 3 contacts with the hole injection layer 4A to the interface wherethe hole injection layer 4A contacts with the buffer layer 6A.

Note that examples have been reported on in the past of the technologyitself of using a layer that includes tungsten oxide crystals as thehole injection layer. For example, Non-Patent Literature 1 suggests thatcrystallizing a tungsten oxide layer by annealing at 450° C. improvesthe hole conduction efficiency. However, Non-Patent Literature 1 doesnot disclose the conditions for forming a tungsten oxide layer with alarge area, nor the effects that tungsten oxide formed above thesubstrate as a hole injection layer has on other layers above thesubstrate. Non-Patent Literature 1 therefore does not demonstrate thepotential for practical mass-production of a large organic EL displaypanel. Furthermore, Non-Patent Literature 1 does not disclose purposelyforming tungsten oxide nanocrystals having the oxygen vacancy structuresor similar structures in the hole injection layer. In contrast, the holeinjection layer according to one aspect of the present invention isformed from a tungsten oxide layer that is resistant to chemicalreactions, is stable, and withstands the mass production process oflarge organic EL panels. Furthermore, purposely incorporating the oxygenvacancy structures or similar structures in the tungsten oxide layerachieves excellent hole conduction efficiency, which is a decisivedifference from conventional technology.

(Electron Injection Layer 7/Cathode 8D/Sealing Layer 9)

The electron injection layer 7 has a function to inject electrons fromthe cathode 8D to the light-emitting layer 6B. It is desirable that theelectron injection layer 7 be, for example, a 5-nm thick layer ofbarium, or a 1-nm thick layer of lithium fluoride or sodium fluoride, ora combination thereof.

The cathode 8D is, for example, composed of an ITO layer with a filmthickness of approximately 100 nm. A direct current power supply DC isconnected to the anode 2 and cathode 8D so as to supply power from anexternal source to the organic EL element 1C.

The sealing layer 9 has a function to seal the organic EL element 1Cfrom being exposed to water or air. The sealing layer 9 is, for example,formed from a material such as silicon nitride (SiN) or siliconoxynitride (SiON). In the case of a top emission type organic ELelement, it is desirable that the sealing layer 9 be formed from alight-transmissive material.

<Method of Manufacturing Organic EL Element 1C>

The following describes an example of a method for manufacturing theentire organic EL display panel 1C, with reference to FIGS. 26A through26C, FIGS. 27A and 27B, FIGS. 28A through 28D, and FIGS. 29A through29D.

First, a thin film of silver is formed by sputtering, for example, onthe substrate 10. The thin film is then patterned by, for example,photolithography to form the anode 2 in a matrix (FIG. 26A). Note thatthe thin film may be formed by another method such as vacuum deposition.

Next, an ITO thin film is formed by sputtering, for example, and the ITOthin film is patterned by photolithography, for example, to form the ITOlayer 3.

Subsequently, a thin film 4X containing tungsten oxide is then formed onan upper surface of a base layer including the anode 2 according to thepredetermined film forming conditions (i.e., the low rate film formingconditions) described below (FIG. 26B). Here, the upper surface of thebase layer corresponds to the upper surface of the ITO layer 3. Byforming the thin film 4X, which corresponds to the hole injection layer4A, in the manner described above, the oxygen vacancy structure isformed therein, and thus an occupied energy level is formed within abinding energy range from 1.8 eV to 3.6 eV lower than the lowest bindingenergy of the valence band. Thus, excellent hole injectioncharacteristics of the hole injection layer 4A is ensured.

Subsequently, atmospheric firing of the thin film 4X is performed at afiring temperature of at least 200° C. to at most 230° C. for aprocessing time of at least 15 minutes to at most 45 minutes; i.e., inthe similar manner as in embodiment 1. By firing the thin film 4X insuch a manner, heat is applied to the thin film 4X, which brings about adensification of the thin film 4X to have an increased film densitywithin a range of at least 5.8 g/cm³ to at most 6.0 g/cm³. By the filmdensity of the thin film 4X being increased through the firing process,the dissolution resistance of the thin film 4X to the etching solution,the cleaning liquid, etc., used in the subsequent bank forming processincreases.

Subsequently, a bank material layer 5X is formed on the thin film 4Xwith bank material composed of organic material. Further, a portion ofthe bank material layer 5X is removed to expose a portion of the thinfilm 4X (FIG. 26C). The bank material layer 5X is formed by applicationor by another method. Here, the removal of the bank material layer 5Xcan be performed by patterning using a predetermined developer (atetramethylammonium hydroxide (TMAH) solution or the like).

Although the thin film 4X has excellent chemical resistance due tohaving undergone the firing process following the forming thereof, thetungsten oxide constituting the thin film 4X dissolves to the TMAHsolution to a certain extent. Specifically, when bank residue remainingadhered onto the surface of the thin film 4X is washed away by using thedeveloper, erosion of the exposed portion of the thin film 4X takesplace. Accordingly, a slight degree of film thickness reduction of thethin film 4X takes place, which results in a concave portion that isconcave towards the direction of the anode 2 being formed in the thinfilm 4X (FIG. 27A). Thus, the hole injection layer 4A is formed to havea concave portion 4 a.

Next, repellency treatment is performed on the surface of the bankmaterial layer 5X using fluorine plasma, for example, to form the banks5. Subsequently, an ink composition containing organic material isdripped, for example using the inkjet method, into a region defined bythe banks 5, and the ink is then dried. Each of the buffer layer 6A andthe light-emitting layer 6B is formed in this way (FIG. 27B). Note thatthe depositing of ink may be performed according to other methods suchas the dispenser method, the nozzle coating method, the spin coatingmethod, intaglio printing, and relief printing.

Next, a thin film of barium constituting the electron injection layer 7is formed by vacuum deposition, for example (FIG. 28A).

Then, an ITO thin film constituting the cathode 8D is formed bysputtering, for example (FIG. 28B).

Next, on the cathode 8D, the sealing layer 9 is formed (FIG. 28C).

This completes the organic EL element 1C.

The following describes the film forming conditions for the holeinjection layer 4A (thin film 4X). It is desirable that the holeinjection layer 4A (thin film 4X) be formed by reactive sputtering.Specifically, metal tungsten is placed in the chamber as the sputteringtarget, and argon gas and oxygen gas are introduced into the chamber asthe sputtering gas and oxygen gas, respectively. Under this condition,the argon in the argon gas is ionized by the application of highvoltage, and the ionized argon is caused to bombard the sputteringtarget. The metal tungsten ejected as a result of the sputteringphenomenon reacts with the oxygen gas to produce tungsten oxide, thusforming a tungsten oxide layer on the ITO layer 3.

To provide a brief explanation of the film forming conditions underwhich the hole injection layer 4A (thin film 4X) is formed, the filmforming conditions are such that: (i) the total pressure of the gasintroduced into the chamber is at least 2.3 Pa and at most 7.0 Pa; (ii)the partial pressure of the oxygen gas is at least 50% and at most 70%of the total pressure of the gas; (iii) the input power density per unitarea of the sputtering target is at least 1.4 W/cm² and smaller than 2.8W/cm²; and (iv) the value yielded by dividing the total pressure by theinput power density is greater than 0.7 Pa·cm²/W. Detailed descriptionof the film forming conditions is provided later. The hole injectionlayer 4A composed of tungsten oxide having the nanocrystal structure isformed under these film forming conditions.

(Another Example of Processing from Anode Forming Process to BankForming Process)

Next, another example of the processing from the anode forming processto the bank forming process is described, with reference to FIGS. 29Athrough 29D and FIGS. 30A through 30C. Note that in this processing,description is provided on an example where a planarizing film 17 isformed on the surface of the substrate 10.

First, a planarizing film 17 is formed on the substrate 10 by using aninsulating resin material such as polyimide or acrylic. With the vapordeposition method, the following three layers are layered sequentiallyon the planarizing film 17: an Al alloy thin film 2X, an IZO thin film3X, and a thin film (tungsten oxide film) 4X (FIG. 29A).Aluminum-cobalt-lanthanum (ACL) material, for example, is used as the Alalloy material.

Next, a resist pattern R is formed by photolithography in the regionabove the substrate 10 in which the three layers of the anode 2, the IZOlayer 3A, and the hole injection layer 4B are to be formed (FIG. 29B).

Next, patterning is performed by dry etching (D/E) the regions of thethin film 4X not covered by the resist pattern R (FIG. 29C). During thisdry etching, only the thin film 4X is selectively etched by using eithera mixture of fluorinated gas and N2 gas, or a mixture of fluorinated gasand O2 gas. The following is an example of the specific settingconditions for the dry etching.

[Conditions for Dry Etching]

Target of treatment: tungsten oxide film

Etching gas: fluorine-containing gas (SF₆, CF₄CHF₃)

Mixed gas: O₂, N,

Gas mixture ratio: CF₄:O₂=160:40

Supplied power: Source 500 W, Bias 400 W

Pressure: between 10 mTorr and 50 mTorr

Etching temperature: room temperature

Performing the above dry etching yields the hole injection layer 4B.Subsequently, ashing is performed with O₂ gas to facilitate removal ofthe resist pattern R during the following wet etching (W/E) process.

Then, the regions of the IZO thin film 3X and the Al alloy thin film 2Xnot covered by the resist pattern R are patterned by wet etching (FIG.29D). By using a mixed solution of nitric acid, phosphoric acid, aceticacid, and water as the etchant, wet etching is performed simultaneouslyon both the IZO thin film 3X and the Al alloy thin film 2X.

The following is an example of the specific setting conditions for thewet etching.

[Conditions for Wet Etching]

Target of treatment: IZO thin film and Al alloy thin film

Etchant: mixed aqueous solution of nitric acid, phosphoric acid, andacetic acid

Blend ratio of solvent: not specified (mixing is possible under typicalconditions)

Etching temperature: lower than room temperature

Note that to perform the wet etching well, it is desirable that the IZOthin film 3X, which is the upper layer among the two layers, have athickness of 20 nm or less. This is because the amount of side etchinggrows large if the thickness exceeds 20 nm.

Here, instead of an IZO layer formed from an IZO thin film, an ITO layerformed from an ITO thin film may of course be used.

The anode 2 and the IZO layer 3A are formed through the above processes.Subsequently, the resist pattern R is removed through a resist removingstep, yielding a patterned triple layer structure composed of the anode2, the IZO layer 3A, and the hole injection layer 4B (FIG. 30A). In thisprocess, the hole injection layer 4B is formed at a locationcorresponding to that of the anode 2 and the IZO layer 3A.

Next, the bank material film 5X is formed on the exposed surface of theplanarizing film 17 (not shown in the figures) and is patterned to formthe banks 5 (FIG. 30B).

Note that following this point, each of the buffer layer 6A and thelight-emitting layer 6B is formed by preparing a predetermined ink asdescribed above, dripping the ink into regions partitioned by the banks5, and drying the ink (FIG. 30C).

<Various Experiments Concerning Film Forming Conditions for HoleInjection Layers 4A, 4B and Observations>

(Film Forming Conditions for Hole Injection Layers 4A, 4B)

In embodiment 2, the hole injection layers 4A and 4B are each providedwith the nanocrystal structure due to the tungsten oxide film thatbecomes the hole injection layer 4A/4B being formed under thepredetermined film forming conditions (the low rate film formingconditions). Accordingly, the hole conduction efficiency of the holeinjection layers 4A and 4B is improved, and further, the organic ELelement 1C can be driven at low voltage. The predetermined film formingconditions are now described in detail.

In the forming of the hole injection layer 4A/4B, a DC magnetronsputtering device was used as the sputtering device, with metal tungstenas the sputtering target. The substrate temperature was not controlled.Further, it is considered desirable to form the tungsten oxide films byreactive sputtering by using argon gas and oxygen gas as the sputteringgas and the reactive gas, respectively, and by introducing the gases atequivalent amounts to flow in the chamber of the sputtering device. Notethat the method of forming the hole injection layer 4A/4B is not limitedto this, and well-known methods other than sputtering may be used forfilm formation, such as vapor deposition or CVD.

In order to form each of the hole injection layers 4A and 4B composed oftungsten oxide to have the nanocrystal structure, the atoms and clustersthat hit the substrate need to reach the substrate with a kinetic energylow enough so as not to damage the orderly structure already formed onthe substrate and to be able to bond together in an orderly manner whilemoving along substrate. It is therefore desirable to use as low a filmforming rate (deposition rate) as possible.

Based on the experiments result described below, assumption is made thatthe above-described film forming conditions (i) through (iv) achieve thelow film forming rate when film deposition is performed by reactivesputtering. The present inventors have confirmed that by forming a holeinjection layer under the film forming conditions (i) through (iv), ahole injection layer composed of tungsten oxide having the nanocrystalstructure is obtained, and the driving voltage of the organic EL elementis reduced.

With respect to condition (i), note that while the upper limit of thetotal pressure in the experiment described below is 4.7 Pa, the presentinventors confirmed separately that a similar trend is exhibited atleast up to 7.0 Pa.

Furthermore, with respect to condition (ii), while the partial pressureof the oxygen gas is set to 50% of the total pressure of gas in theexperiment described below, the present inventors confirmed thereduction in driving voltage at least while the partial pressure of theoxygen gas is in a range of at least 50% and at most 70% of the totalpressure of gas.

A further explanation of condition (iv) is now provided. When the flowamounts of argon gas and oxygen gas in the chamber are equivalent, it isassumed that film properties are determined by the input power densityand the total pressure. The input power density in (iii) changes boththe number and kinetic energy of tungsten atoms and tungsten clustersreleased from the target by sputtering. In other words, lowering theinput power density reduces the number of tungsten atoms released fromthe sputtering target and also lowers the kinetic energy. As a result,less tungsten reaches the substrate, arriving with a low kinetic energy.Hence, lowering the input power density should allow for film formationat a low film forming rate. Furthermore, the total pressure in (i)changes the mean free path of the tungsten atoms and tungsten clustersreleased from the sputtering target. In other words, if the totalpressure is high, the tungsten atoms and tungsten clusters have a higherprobability of repeatedly colliding with the gas in the chamber beforereaching the substrate. The directions of arrival of the tungsten atomsand tungsten clusters thus become scattered, and kinetic energy is lostdue to the collisions. As a result, less tungsten reaches the substrate,arriving with a low kinetic energy. Hence, raising the total pressureshould allow for film formation at a low film forming rate.

It is considered, however, that there are limits to changing the filmforming rate by independently controlling the input power density andthe total pressure. Accordingly, the value yielded by dividing the totalpressure by the input power density was adopted as a new parameterdetermining the film forming rate. This new parameter constitutes filmforming condition (iv).

Specifically, the above-described parameter (total pressure/powerdensity) for forming the nanocrystal structure in embodiment 2 is atleast 0.78 Pa·cm²/W in the range of the experiment described below.Further, a value larger than 0.7 Pa·cm²/W is considered acceptable, andfor even more reliable film formation, a value of 0.8 Pa·cm²/W orgreater is considered desirable. On the other hand, the upper limit forthe parameter is 3.13 Pa·cm²/W in the range of the experiment describedbelow. Further, a value smaller than 3.2 Pa·cm²/W is consideredacceptable, and for even more reliable film formation, a value of 3.1Pa·cm²/W or less is desirable. Based on the above observation of thefilm forming rate and the nanocrystal structure, however, a lower filmforming rate is considered more desirable, and therefore restrictionsare not necessarily placed on the upper limit for the parameter. Filmforming condition (iv) was determined based on the above considerations.

The present inventors confirmed in a separate experiment that for highervalues of the above parameter, the film forming rate is lower, whereasfor lower values of the above parameter, the film forming rate ishigher.

Next, the present inventors confirmed the validity of the above filmforming conditions through experiments.

First, hole-only devices 1D illustrated in FIG. 25 were manufactured asassessment devices in order to assess the degree to which the holeconduction efficiency of the hole injection layers 4A and 4B depends onfilm forming conditions. As already explained in embodiment 1, since itcould be considered that only holes function as a carrier in a hole-onlydevice, a hole-only device is ideal in making an assessment of holeconduction efficiency

As illustrated in FIG. 25, the hole-only devices 1D were prepared bymodifying the organic EL element 1C illustrated in FIG. 24 to have astructure of an assessment device. More specifically, the ITO cathode 8Dwas replaced with a cathode 8E composed of Au, the anode 2 was removedand the ITO layer 3 was used as the anode instead, and the electroninjection layer 7 and the banks 5 were removed. The hole-only devices 1Dwere prepared according to the manufacturing method described above.Further, the film thickness of each of the layers in the hole-onlydevices 1D were set such that the hole injection layer 4A has a filmthickness of 30 nm, the buffer layer 6A composed of TFB has a filmthickness of 20 nm, the light-emitting layer 6B composed of F8BT has afilm thickness of 70 nm, and the cathode 8E composed of Au has a filmthickness of 100 nm.

In the manufacturing of the hole-only devices 1D, the hole injectionlayer 4A was formed by reactive sputtering in a DC magnetron sputteringdevice. The gas introduced into the chamber was composed of at least oneof argon gas and oxygen gas, and the sputtering target used was metaltungsten. Further, the substrate temperature was not controlled, and thetotal pressure was adjusted by controlling the flow amount of each gas.The partial pressure of each of the argon gas and the oxygen gas withrespect to the total pressure of the chamber gas was set to the samevalue of 50%.

The hole-only devices 1D were manufactured with the hole injection layer4A formed under five different film forming conditions α through ε shownin Table 7. Hereinafter, the hole-only device 1D formed under filmforming conditions α is referred to as HOD-α, the hole-only device 1Dformed under film forming conditions β is referred to as HOD-β, thehole-only device 1D formed under film forming conditions γ is referredto as HOD-γ, the hole-only device 1D formed under film formingconditions δ is referred to as HOD-δ, and the hole-only device 1D formedunder film forming conditions ε is referred to as HOD-ε.

TABLE 7 Film Total Oxygen Input Total Pressure/ Forming Pressure PartialPressure Power Density Power Density Condition (Pa) (%) (W/cm²) (Pa ·cm²/W) α 4.70 50 1.50 3.13 β 4.70 50 3.00 1.57 γ 4.70 50 6.00 0.78 δ2.35 50 1.50 1.57 ε 2.35 50 6.00 0.39

The hole-only devices 1D so prepared were then connected to the directcurrent power supply DC, and voltage was applied thereto. Furthermore,different voltages were applied to the hole-only devices 1D, and currentvalues for different voltage values were measured. Further, the currentvalues were converted into current values per unit surface area of thedevices (current density values).

FIG. 31 is a diagram illustrating the relation between the appliedvoltages and the current density values of the hole-only devices 1D. InFIG. 31, the vertical axis indicates current density (mA/cm²), whereasthe horizontal axis indicates applied voltage (V).

Table 8 shows the driving voltage for each of the hole-only devices 1D.Note that hereinafter, the expression “driving voltage” refers to anapplied voltage for a current density value of 0.3 mA/cm².

It can said that the hole conduction efficiency of the hole injectionlayer 4A is higher in hole-only devices having smaller driving voltages.This is since the structure of each hole-only device 1D other than thehole injection layer 4A is the same, and the hole injection barrierbetween adjacent layers, excluding the hole injection layer 4A, as wellas the hole conduction efficiency of each layer, again excluding thehole injection layer 4A, is assumed to be constant between the hole-onlydevices 1D. Further, it is assumed that the hole conduction efficiencyof the hole injection layer 4A more strongly influences thecharacteristics of the hole-only devices 1D than the hole injectionefficiency from the hole injection layer 4A to the buffer layer 6A. Thereasons for this are presented later. Further, through a separateexperiment, the present inventors confirmed that the Schottky ohmiccontact pertaining to the present invention is formed between the ITOlayer 3 and the hole injection layer 4A in the hole-only devices 1D, ina similar manner as described in embodiment 1. Accordingly, thedifferences in driving voltages of the hole-only devices 1D, whichderive from the different film forming conditions for forming the holeinjection layer 4A, are considered as strongly reflecting the differencein hole conduction efficiency of the hole injection layer 4A between thehole-only devices 1D.

TABLE 8 Sample Name Driving Voltage (V) HOD-α 6.25 HOD-β 7.50 HOD-γ 8.50HOD-δ 8.50 HOD-ε 9.49

As shown in Table 8 and FIG. 31, the current density-applied voltagecurve rises the slowest for the HOD-ε, which has the highest drivingvoltage among the elements. Accordingly, it is inferred that HOD-α, β,β, and δ have superior hole conduction efficiency as compared to HOD-ε,which is manufactured under the film forming conditions with a low totalpressure and the maximum input power density.

Thus far, tests on the hole conduction efficiency of the hole injectionlayer 4A in the hole-only devices 1D have been described. Except for thecathode 8E, the hole-only devices 1D each have the same structure as theorganic EL element 1C particularly in terms of essential parts of thestructure influencing device characteristics. Accordingly, in theorganic EL element 1C as well, the dependence of the hole conductionefficiency of the hole injection layer 4A on the film forming conditionsis essentially the same as in the hole-only devices 1D.

In order to confirm this point, a plurality of organic EL elements 1Cwere prepared each using a hole injection layer 4A formed under adifferent one of film forming conditions α through ε. Hereinafter, theorganic EL element 1C formed under film forming conditions α is referredto as BPD-α, the organic EL element 1C formed under film formingconditions β is referred to as BPD-β, the organic EL element 1C formedunder film forming conditions γ is referred to as BPD-γ, the organic ELelement 1C formed under film forming conditions δ is referred to asBPD-δ, and the organic EL element 1C formed under film formingconditions ε is referred to as BPD-ε.

Each of the organic EL elements 1C were prepared by modifying theorganic EL element 1C in FIG. 24 to have a structure of an assessmentdevice. More specifically, the cathode 8D was formed of aluminum insteadof ITO, the anode 2 was removed and the ITO layer 3 was used as theanode instead, and the banks 5 were removed. The organic EL elements 1Cwere prepared according to the manufacturing method described above.Further, the film thickness of each of the layers in the organic ELelements 1C were set such that the hole injection layer 4A has a filmthickness of 30 nm, the buffer layer 6A composed of TFB has a filmthickness of 20 nm, the light-emitting layer 6B composed of F8BT has afilm thickness of 70 nm, the electron injection layer 7 composed of abarium layer has a film thickness of 5 nm, and the cathode 8 composed ofan aluminum layer has a film thickness of 100 nm.

The organic EL elements 1C prepared according to the different filmforming conditions α through ε were connected to the direct currentpower source DC, and voltage was applied thereto. Furthermore, differentvoltages were applied to the organic EL elements 1C, and current valuesfor different voltage values were measured. Further, the current valueswere converted into current values per unit surface area of the devices(current density values).

FIG. 32 illustrates the relation between the applied voltages and thecurrent density values of the organic EL elements 1C. In FIG. 32, thevertical axis indicates current density (mA/cm²), whereas the horizontalaxis indicates applied voltage (V).

Table 9 shows the driving voltage for each of the organic EL elements1C. Note that hereinafter, the expression “driving voltage” refers to anapplied voltage for a current density value of 8 mA/cm².

TABLE 9 Sample Name Driving Voltage (V) BPD-α 9.25 BPD-β 11.25 BPD-γ11.50 BPD-δ 12.25 BPD-ε 14.00

As shown in Table 9 and FIG. 32, the current density-applied voltagecurve rises the slowest for the BDP-ε, which has the highest drivingvoltage among the elements. This trend is similar to the trend observedin the hole-only devices HOD-α through HOD-ε, which were prepared underthe same respective film forming conditions.

From the above results, it was confirmed that in the organic EL elements1C as well, the hole conduction efficiency of the hole injection layer4A depends on the film forming conditions, similar to the case of thehole-only devices 1D. Specifically, it is inferred that in the organicEL elements 1C as well, forming the film under the specific rangesdefined by the film forming conditions α, β, γ, and δ improves the holeconduction efficiency of the hole injection layer 4A, thereby achievinga low driving voltage.

Note that among the above conditions, the condition concerning inputpower is represented in terms of input power density, as indicated inTable 7. When using a DC magnetron sputtering device that is differentfrom the one used in the present experiment, a hole injection layercomposed of a tungsten oxide layer with an excellent hole conductionefficiency, as in the present experiment, can be yielded by adjustingthe input power so that the input power density fulfills the abovecondition. Further, among the above-described film forming conditions,the condition concerning the total pressure of gas and the conditionconcerning the partial pressure of oxygen gas remain the same regardlessof the device used.

Additionally, when forming the hole injection layer 4A by reactivesputtering in the sputtering device, no deliberate adjustment ofsubstrate temperature is performed in the sputtering device, which isassumed to be placed under room temperature. Therefore, the substratewas at room temperature at least before the forming of the holeinjection layer 4A. However, while forming of the hole injection layer4A is being performed, there is a possibility that the substratetemperature may rise by several tens of degrees Celsius.

Note that through a separate experiment, the present inventors confirmedthat when the partial pressure of the oxygen gas is increasedexcessively, the driving voltage conversely increases. Accordingly, itis desirable for the partial pressure of the oxygen gas to be within arange of at least 50% and at most 70% of the total pressure of gas.

The above experiment results indicate that in terms of low voltagedrive, an organic EL element provided with a hole injection layermanufactured under film forming conditions α, ε, γ, and δ is desirable,and that an organic EL element manufactured under film formingconditions α and β is even more desirable. Hereinafter, an organic ELelement provided with a hole injection layer manufactured under filmforming conditions α, ε, γ, or δ is the target of the presentdisclosure.

(Chemical State of Tungsten in Hole Injection Layer 4A)

Tungsten atoms with a valence of five exist in the tungsten oxide layerconstituting the hole injection layers 4A and 4B in embodiment 2. Thetungsten atoms with a valence of five are formed by adjusting the filmforming conditions as shown in the above experiments. Details concerningthis point are provided in the following.

In order to confirm the chemical state of the films of tungsten oxideformed under the above film forming conditions α through ε, a hard X-rayphotoelectron spectroscopy measurement (hereinafter simply referred toas “HXPS measurement”) experiment was performed. Typically, the opticalspectrum yielded by hard X-ray photoelectron spectroscopy (hereinaftersimply referred to as “HXPS spectrum”) reveals information for up to afilm thickness of a few dozen nanometers in the object being measured.In other words, bulk information on the film is obtained, and themeasurement depth is determined by the angle between the normal line tothe surface and the direction in which photoelectrons are to bedetected. In the present experiment, this angle was adjusted to be 40°to allow for observation of the valence state in the entirety of thetungsten oxide layer in the thickness direction.

The conditions under which the HXPS measurement was conducted are asfollows. Note that no charge-up occurred during measurement.

(HXPS Measurement Conditions)

BL47XU beamline of SPring-8 was used.

Light source: synchrotron radiation (energy of 8 keV)

Bias: None

Electron emission angle: angle of 40° with normal line to the substrate

Interval between measurement points: 0.05 eV

Samples for HXPS measurement were manufactured under the film formingconditions α through ε shown in Table 7. A tungsten oxide layer(considered to be the hole injection layer 4A) was formed to a thicknessof 30 nm by reactive sputtering on an ITO substrate formed on glass. Theresult was taken as the sample for HXPS measurement. The samples forHXPS measurement manufactured under the film forming conditions α, β, γ,δ, and ε are hereinafter respectively referred to as sample α, sample β,sample γ, sample δ, and sample ε.

HXPS measurement was performed on the hole injection layer 4A in each ofthe samples α through ε. FIG. 33 is a diagram illustrating the resultingspectra. The origin of the horizontal axis, which represents bindingenergy, corresponds to the Fermi level of the ITO substrate, and theleft direction with respect to the origin is positive. The vertical axisrepresents photoelectron intensity.

Three peaks can be observed in the binding energy range shown in FIG.33. From left to right in FIG. 33, the peaks belong to the followingenergy levels of tungsten: 5p_(3/2) (W5p_(3/2)), 4f_(5/2) (W4f_(5/2)),and 4f_(7/2) (W4f_(7/2)).

Next, peak fitting analysis was performed on the peaks belonging toW5p_(3/2), W4f_(5/2), and W4f_(7/2) in the spectrum of each sample,using XPSPEAK 4.1, which is software for photoelectron spectroscopyanalysis. First, based on the photoionization cross-section for the hardX-ray energy, the area intensity ratio of each component correspondingto W4f_(7/2), W4f_(5/27), and W5p_(3/2) was fixed as follows:W4f_(7/2):W4f_(5/2):W5p_(3/2)=4:3:10.5. Next, as shown in Table 10, theposition of the peak top of the W4f_(7/2) component with a valence ofsix (W⁶⁺4f_(7/2)) was aligned with a binding energy of 35.7 eV. Thelocation of the peak top and the initial value of the full width at halfmaximum were set within the range shown in Table 10 for the componentbelonging to the surface photoelectrons of W5p_(3/2), W4f_(5/2),W4f_(7/2), the component belonging to the valence of six, and thecomponent belonging to the valence of five. In the Gaussian-Lorentzianmixed function used for fitting of the components, the initial value ofthe ratio in the Lorentzian function was also set within the rangeindicated in Table 10. Furthermore, the initial value of the areaintensity of each component was set freely while maintaining the aboveintensity ratio. Optimization calculations were performed a maximum of100 times by varying the area intensity for each component whilemaintaining the above intensity ratio, and by varying the peak location,the full width at half maximum, and the ratio in the Lorentzian functionfor each component within the ranges indicated in Table 10. Thesecalculations yielded the final peak fitting analysis results.

TABLE 10 Corresponding W5p_(3/2) W4f_(5/2) W4f_(7/2) peakW^(sur)5p_(3/2) W⁶⁺5p_(3/2) W⁵⁺5p_(3/2) W^(sur)4f_(5/2) W⁶⁺4f_(5/2)W⁵⁺4f_(5/2) W^(sur)4f_(7/2) W⁶⁺4f_(7/2) W⁵⁺4f_(7/2) Peak Energy 42.30 to41.20 to 39.70 to 38.75 to 37.80 to 36.72 to 36.60 to 35.70 34.60 to(eV) 43.07 41.30 38.65 39.13 37.85 36.95 36.90 (reference) 34.80 Valueof full 1.73 to 1.93 to 1.8 to 1.40 to 0.87 to 0.90 to 1.40 to 0.87 to0.90 to width at half 2.40 2.24 2.86 1.60 0.98 1.50 1.60 0.98 1.50maximum (eV) Lorentzian 0 13 to 40 0 to 25 0 to 57 0 to 6 0 to 20 0 to57 0 to 6 0 to 20 function ratio (%)

FIGS. 34A and 34B show the final peak fitting analysis results. FIG. 34Ashows the analysis results for sample α, and FIG. 34B shows the analysisresults for sample ε.

In both FIGS. 34A and 34B, the dashed lines (sample α, sample ε) areactual measured spectra (corresponding to the spectra in FIG. 33), thelines with alternate long and two short dashes (surface) are thecomponent belonging to the surface photoelectrons (W^(sur)5p_(3/2),W^(sur)4f_(5/2), W^(sur)4f_(7/2)), the dotted lines (W⁶⁺) are thecomponent belonging to the valence of six (W⁶⁺5p_(3/2), W⁶⁺4f_(5/2),W⁶⁺4f_(7/2)), and the alternating long and short dashed lines (W⁵⁺) arethe component belonging to the valence of five (W⁵⁺5p_(3/2),W⁵⁺4f_(5/2), W⁵⁺4f_(7/2)). The solid lines (fit) are the spectra yieldedby summing the components indicated by the lines with alternate long andtwo short dashes and the alternating long and short dashed lines.

The spectra for the dashed lines and the solid lines in FIGS. 34A and34B match extremely well. In other words, the peaks belonging to theenergy levels of W5p_(3/2), W4f_(5/2), and W4f_(7/2) can all bedescribed well by the sum of the component (surface) belonging to thephotoelectrons from the surface of the hole injection layer 4A and thecomponent (W⁶⁺) belonging to a valence of six, as well as the component(W⁵⁺) belonging to a valence of five, included within the hole injectionlayer 4A.

Furthermore, in the binding energy range that is between 0.3 eV and 1.8eV lower than the component belonging to a valence of six (W⁶⁺) insample α of FIG. 34A, the existence of a corresponding componentbelonging to a valence of five (W⁵⁺) can be confirmed. By contrast, insample E of FIG. 34B, no such component belonging to a valence of fivecan be confirmed. For the purposes of illustration, the circled regionin each of FIGS. 34A and 34B is shown enlarged to the right. As seen inthe enlarged diagrams, a ridge in the alternating long and short dashedline for W⁵⁺ (labeled (c) in FIG. 34A) can be clearly observed forsample α, but no such ridge can be observed for sample E. Furthermore,looking more closely at the enlarged diagrams, the solid line (fit),which is the sum of the components resulting from peak fitting, exhibitsa large “shift” in sample α with respect to the dotted line (W⁶⁺), whichcorresponds only to the component with a valence of six. In sample E,however, the “shift” is not as large as in sample α. In other words, the“shift” in sample α can be inferred as suggestive of the existence oftungsten atoms with the valence of five.

Next, the ratio of tungsten atoms with a valence of five to tungstenatoms with a valence of six, i.e. W⁵⁺/W⁶⁺, was calculated for samples αthrough ε. This ratio was calculated by dividing the area intensity ofthe component belonging to a valence of five by the area intensity ofthe component belonging to a valence of six in the peak fitting analysisresults for each sample.

Note that the area intensity ratio of the component belonging to avalence of five to the component belonging to a valence of six should bethe same for W5p_(3/2), W4f_(5/2), and W4f_(7/2) based on the principleof measurement. In the present experiment, it was confirmed that thesevalues are indeed the same. Therefore, the following analysis onlyrefers to W4f_(7/2).

Table 11 shows the ratio W⁵⁺/W⁶⁺ in W4f_(7/2) for samples α through ε.

TABLE 11 Sample Name W⁵⁺/W⁶⁺ Sample α 7.4% Sample β 6.1% Sample γ 3.2%Sample δ 3.2% Sample ε 1.8%

Based on the values of W⁵⁺/W⁶⁺ shown in Table 11, the sample with thelargest ratio of tungsten atoms with a valence of five in the holeinjection layer 4A is sample α. The ratio then tends to decrease in theorder of sample β, sample γ, and sample δ, with sample ε having thelowest ratio. Comparing the results from Table 9 and Table 11, it isclear that as the ratio of tungsten atoms with a valence of five in thehole injection layer 4A increases, the driving voltage of the organic ELelements tends to be lower.

Using the above HXPS measurement to calculate the composition ratio oftungsten to oxygen, it was confirmed that the ratio of the number oftungsten atoms to the number of oxygen atoms in the hole injection layer4A was nearly 1:3 on average throughout the layer in all of the samplesα through ε. Based on this ratio, it can be assumed that in all of thesamples α through ε, nearly the entire hole injection layer 4A has abasic structure with atomic coordinates based on tungsten trioxide. Notethat the present inventors performed X-ray absorption fine structure(XAFS) measurement with respect to the hole injection layer 4A of bothsamples α and ε and confirmed that the above basic structure is formedtherein.

(Electronic State of Hole Injection Layer 4A)

The hole injection layer 4A composed of tungsten oxide in embodiment 2has the occupied energy level near the Fermi surface, similar to thehole injection layer 4 in embodiment 1. Due to the effect of theoccupied energy level, the interface energy level is aligned between thehole injection layer 4A and the buffer layer 6A, thereby reducing thehole injection barrier between the hole injection layer 4A and thebuffer layer 6A. As a result, the organic EL element pertaining toembodiment 2 can be driven at low voltage.

As described below, this occupied energy level near the Fermi surfaceexists not only at the above interface, but also throughout the holeinjection layer 4A at the grain boundaries of the nanocrystals, thusserving as a conduction path for holes. This provides the hole injectionlayer 4A with an excellent hole conduction efficiency, so that theorganic EL element pertaining to embodiment 2 can be driven at a lowervoltage.

An experiment to confirm the existence of the occupied energy level nearthe Fermi surface was performed using UPS measurement on the holeinjection layer 4A in each of the above samples α through ε.

The forming of the hole injection layer 4A of each of the samples wasperformed inside a sputtering device. Then, to prevent atmosphericexposure, the samples α through ε were transported to a glovebox whichwas connected to the sputtering device and which was filled withnitrogen gas. Subsequently, the sample devices were sealed insidetransfer vessels in the glovebox, and were mounted on a photoelectronspectroscopy device. After formation, the hole injection layer 4A wastherefore not exposed to the atmosphere at the point when the UPSmeasurement was performed.

The conditions under which the UPS measurement was conducted are asfollows. Note that charge-up did not occur during measurement.

Light source: He I line

Bias: None

Electron emission angle: Direction of normal line to the substrate

Interval between measurement points: 0.05 eV

FIG. 35 is a diagram illustrating a UPS spectrum of the hole injectionlayer 4A in samples α and ε within area y. In FIG. 35, labels such asarea y and point (iii) are as described in embodiment 1, and thehorizontal axis represents relative binding energy with point (iii) asthe origin.

As illustrated in FIG. 35, the spectral protrusion near the Fermisurface described in embodiment 1 was confirmed in the hole injectionlayer 4A of sample α in a binding energy range from 3.6 eV lower thanpoint (iii) to 1.8 eV lower than point (iii), point (iii) being thelocation at which the valence band rises. On the other hand, such aspectral protrusion was not confirmed in sample ε. Note that in samplesβ, γ, and δ as well, the existence of the spectral protrusion wasconfirmed and the spectral protrusion had a shape and normalizedintensity differing only little from that found in the hole injectionlayer 4A of sample α.

UPS measurement, however, is only an assessment of the surface portionof a measurement target. Upon attempting to confirm the existence of thespectral protrusion near the Fermi surface throughout the entire holeinjection layer 4A by performing HXPS measurement on the hole injectionlayer 4A in samples α and ε, the spectral protrusion was confirmed insample α, as expected, but not in sample ε.

The above experiment proved that the hole injection layer 4A inembodiment 2 has the occupied energy level near the Fermi surface.Therefore, due to a tungsten oxide layer having the upward protrusion(not necessarily a peak) in a binding energy range area approximatelybetween 1.8 eV and 3.6 eV lower than point (iii) in the photoelectronspectrum, i.e. a tungsten oxide layer having the occupied energy levelnear the Fermi surface, being adopted as the hole injection layer, theorganic EL element pertaining to embodiment 2 exhibits excellent holeconduction efficiency.

Here, the characteristics of the hole-only devices and the organic ELelements described in embodiment 2 are thought to be affected more bythe hole conduction efficiency in the hole injection layer 4A than bythe hole injection efficiency from the ITO layer 3 to the hole injectionlayer 4A and the hole injection efficiency from the hole injection layer4A to the buffer layer 6A. The reasons are as follows.

In hole injection layers 4A formed under the film forming conditions α,β, γ, and δ, the spectral protrusion near the Fermi surface wasconfirmed by UPS measurement, as described above. In terms of FIG. 14 inembodiment 1, this means that in these hole injection layers 4A, theinjection sites x exist at a number density sufficient for the injectionsites x to be confirmed by UPS measurement. Furthermore, the shape andthe normalized intensity of the spectral protrusion did not differgreatly between the hole injection layers 4A formed under conditions α,β, γ, and δ. Accordingly, the number density of the injection sites xcan be assumed to be equivalent in the hole injection layers 4A formedunder conditions α, β, γ, and δ. Considering that the film formingcondition α is equivalent to the above film forming condition A inembodiment 1, the hole injection layers 4A formed under conditions α, β,γ, and δ can all be assumed to have a sufficient number density ofinjection sites x with respect to the number density of injection sitesy in the buffer layer 6A. In other words, the hole injection layers 4under conditions α, β, γ, and δ are all considered to have the samelevel of hole injection efficiency from the hole injection layer 4A tothe buffer layer 6A.

However, although the driving voltages of HOD-α, β, γ, and δ in Table 8are all good, there is a spread of 2.25 V therebetween. Therefore, itreasonably follows that a factor other than the hole injectionefficiency from the hole injection layer 4A to the buffer layer 6A isresulting in the spread. Since the Schottky ohmic contact is formedbetween the ITO layer 3 and the hole injection layer 4A in embodiment 2as described above, the remaining factor resulting in the spread can beassumed to be the hole conduction efficiency of the hole injection layer4A itself.

(Analysis of Relationship Between Value of W⁵⁺/W⁶⁺ and Hole ConductionEfficiency)

FIG. 36 is a diagram illustrating the tungsten oxide crystal structure.In the tungsten oxide of embodiment 2, as described above, thecomposition ratio of tungsten to oxygen is approximately 1:3. Therefore,tungsten trioxide is described here as an example.

As shown in FIG. 36, a tungsten trioxide crystal has a structure inwhich six oxygen atoms and one tungsten atom bind in octahedralcoordination, with octahedrons sharing oxygen atoms at vertices. (Tosimplify the illustration, FIG. 36 shows octahedrons in perfectalignment, like rhenium trioxide. In reality, the octahedrons are in aslightly distorted arrangement).

These tungsten atoms that are bound to six oxygen atoms in octahedralcoordination are tungsten atoms with a valence of six. On the otherhand, tungsten atoms with a lower valence than six correspond tostructures in which the octahedral coordination has somehow becomedisrupted. A typical example is when one of the six oxygen atomsescapes, forming an oxygen vacancy structure. In this case, the tungstenatom bonded with the remaining five oxygen atoms has a valence of five.

Typically, when an oxygen vacancy structure exists in a metal oxide, anelectron left behind by the oxygen atom that has escaped is provided toa metal atom near the oxygen vacancy structure in order to maintainelectric neutrality. The valence of the metal atom thus lowers. Anelectron is thus provided in this way to the tungsten atom with avalence of five. This electron is assumed to combine with the electronthat was used to bond with the oxygen atom that escaped, thus forming alone pair of electrons.

Based on the above analysis, the mechanism for hole conduction in thehole injection layer 4A in embodiment 2, which has tungsten atoms with avalence of five, is as follows, for example.

A tungsten atom with a valence of five can supply a hole with anelectron from its own lone pair of electrons. Accordingly, if tungstenatoms with a valence of five exist within a certain distance from eachother, a hole can hop between the lone pair of electrons of tungstenatoms with a valence of five due to the voltage applied to the holeinjection layer. Furthermore, if the tungsten atoms with a valence offive are nearly adjacent, the overlap between 5d orbitals correspondingto the lone pairs of electrons grows large, so that holes can moveeasily without hopping.

In other words, in embodiment 2, holes are thought to be conductedbetween the tungsten atoms with a valence of five that exist in the holeinjection layer 4A.

Based on the above inference, if the value of W⁵⁺/W⁶⁺ is high, i.e. ifthe proportion of tungsten atoms with a valence of five is high in thehole injection layer 4A, as in sample α, tungsten atoms with a valenceof five are more likely to be close or adjacent, thus facilitating holeconduction at low voltage. This provides the organic EL element 1C withexcellent hole conduction efficiency.

Note that samples γ, and δ, for which the value of W⁵⁺/W⁶⁺ is not ashigh as in sample α being approximately 3.2%, were able to driveexcellently at low voltage. It is therefore considered sufficient forthe value of W⁵⁺/W⁶⁺ to be approximately 3.2% or greater.

(Microstructure of Tungsten Oxide in Hole Injection Layer 4A)

The nanocrystal structure exists in the tungsten oxide layerconstituting the hole injection layer 4A in embodiment 2. Thisnanocrystal structure is formed by adjusting the film formingconditions. Details concerning this point are provided in the following.

In order to confirm whether or not the nanocrystal structure exists inthe hole injection layer 4A formed under each of the film formingconditions α through ε in Table 7, a transmission electron microscope(TEM) measurement experiment was performed.

The hole injection layers 4A in the samples for TEM measurement wereformed with a DC magnetron sputtering device. Specifically, a tungstenoxide layer (considered to be the hole injection layer 4A) was formed toa thickness of 30 nm by reactive sputtering on an ITO substrate formedon glass.

The samples for TEM measurement manufactured under the film formingconditions α, β, γ, δ, and ε are hereinafter respectively referred to assample α, sample β, sample γ, sample δ, and sample ε.

Typically, TEM measurement is performed on a surface of a sampleprepared to have small thickness. In embodiment 2, a cross-section ofthe hole injection layer 4A was designated as the target formeasurement. A cross-section was manufactured by using a focused ionbeam (FIB) device to process the sample, and the thickness of the laminawas adjusted to approximately 50 nm. The conditions for FIB processingand TEM measurement are as follows.

(Conditions for FIB Processing)

Device used: Quanta 200 (manufactured by FEI Company)

Accelerating voltage: 30 kV (final voltage: 5 kV)

Lamina thickness: approximately 50 nm

(Conditions for TEM Measurement)

Device used: Topcon EM-002B (manufactured by Topcon TechnohouseCorporation)

Measurement method: high-resolution electron microscopy

Accelerating voltage: 200 kV

FIG. 37 is a diagram illustrating a TEM measurement photograph of across-section of the hole injection layer 4A in samples α through ε. Themagnification ratio in each photograph is as indicated by the scale barshown in each photograph. The photographs are shown with 256 gradationsfrom the darkest to the brightest region.

In each TEM photograph for samples α, β, γ, and δ, regular linearstructures formed due to some of the bright regions in the TEMphotograph being aligned in the same direction can be observed. Asindicated by the scale bar, the linear structures are aligned withintervals of approximately between 1.85 angstroms and 5.55 angstromstherebetween. On the other hand, the bright regions are scatteredirregularly in sample ε, with no regular linear structures beingobservable.

Typically, in a TEM photograph, regions with the above linear structuresrepresent one microscopic crystal. In the TEM pictures in FIG. 37, thesize of these crystals can be seen to be approximately between 5 and 10nm. Therefore, the absence or presence of the above linear structure canbe interpreted as follows: whereas the nanocrystal structure can beconfirmed in the tungsten oxide of samples α, β, γ, and δ, thenanocrystal structure cannot be confirmed in sample ε, which is thoughtto have the amorphous structure nearly throughout the sample.

In the TEM photograph of sample α in FIG. 37, one of the nanocrystals,chosen arbitrarily, is outlined with a white line. Note that thisoutline is not precise, but rather is meant to be an example. This isbecause the TEM photograph shows not only the uppermost surface in thecross-section, but also the conditions lower in the layer, thus makingit difficult to precisely identify the outline. The size of the outlinednanocrystal appears to be approximately 5 nm.

FIG. 38 shows the results of a 2D Fourier transform on the TEMmeasurement photographs in FIG. 37 (hereinafter referred to as 2DFourier transform images). The 2D Fourier transform images are adistribution of the wavenumbers in the reciprocal space of the TEMmeasurement photographs in FIG. 37 and therefore indicate theperiodicity of the TEM measurement photographs. The 2D Fourier transformimages in FIG. 38 were created by performing a Fourier transform on theTEM photographs of FIG. 37 using LAview Version #1.77, which is imageprocessing software.

In the 2D Fourier transform images for samples α, β, γ, and δ, brightregions formed by two or three concentric circles centering on thecenter point (δ point) can be seen relatively clearly. On the otherhand, in sample ε, such a bright region formed by concentric circles isnot clear.

The lack of clarity of the bright region formed by concentric circlesindicates the loss of order in the TEM photographs of FIG. 37. In otherwords, the hole injection layer 4A in samples α, ε, γ, and δ, in whichthe bright region formed by concentric circles is clear, indicates arelatively high level of regularity and orderliness in the layer,whereas the hole injection layer 4A in sample ε has a low level ofregularity and orderliness.

To clearly express this orderliness, graphs showing the change inluminance versus the distance from the center of the image were createdfor each 2D Fourier transform image in FIG. 38. FIGS. 39A and 39B showan outline of the method of creating the graphs, using sample α as anexample.

As illustrated in FIG. 39A, a 2D Fourier transform image was rotated 1°at a time from 0° to 359°, with the center point as the center ofrotation. Upon every degree of rotation, the luminance versus thedistance from the center point along the X-axis was measured.Measurement results for each degree of rotation were then summed up anddivided by 360, yielding the average luminance versus the distance fromthe center point (hereinafter referred to as normalized luminance). FIG.39B shows a plot with distance from the center point along thehorizontal axis and normalized luminance for each distance along thevertical axis. Microsoft Office Picture Manager was used to rotate the2D Fourier transform images, and the image processing software ImageNoswas used to measure the distance from the center point and theluminance. Hereinafter, the plot showing the relation between thedistance from the center point and the normalized luminance at eachdistance, as created with the method described in FIGS. 39A and 39B, isreferred to as a “plot of change in luminance”.

FIGS. 40 and 41 illustrate the plots of change in luminance for samplesα through E. Apart from a high luminance region at the center point,each of the samples exhibited a peak as indicated by the arrows.Hereinafter, the peak indicated by the arrow nearest the center point inthe plot of change in luminance is referred to as a “peak P1”.

FIGS. 40 and 41 show that as compared to the peak P1 in sample ε, thepeak P1 in samples α, β, γ, and δ is a pointed, convex shape. Thesharpness of the peak P1 in each sample was quantified for comparison.FIG. 42 shows an outline of the method of assessment, using samples αand ε as examples.

Portions (a) and (b) in FIG. 42 are plots of change in luminance forsamples α and E respectively. Portions (a1) and (b1) in FIG. 42 areenlarged diagrams of each peak P1 and the surrounding region. The “L” inthe figures represents the “peak width L of the peak P1” and is used asan index of how “pointed” the peak P1 is.

In order to more accurately determine this “peak width L of the peakP1”, the first derivative of the plot of change in luminance in portions(a1) and (b1) is shown in portions (a2) and (b2) in FIG. 42. In portions(a2) and (b2) in FIG. 42, the peak width L is the difference between thevalue along the horizontal axis corresponding to the peak top of thepeak P1 and the value along the horizontal axis, in the direction of thecenter point from the peak, corresponding to the position at which thederivative first becomes zero.

Table 12 shows the values of the peak width L in samples α through ε,with the value along the horizontal axis corresponding to the peak topof the peak P1 normalized as 100.

TABLE 12 Sample Name Peak Width L Sample α 16.7 Sample β 18.1 Sample γ21.3 Sample δ 21.9 Sample ε 37.6

As shown in Table 12, the peak width L is smallest for sample α andincreases in the order of samples β, γ, and δ, with sample ε having thelargest value. The peak width for samples γ and δ is not as small as forsample α. However, even with a value of 21.9, the organic EL element 1Chaving the hole injection layer 4A formed under film forming conditionsγ and δ achieves good hole conduction efficiency, as described above.

The value of the peak width L in Table 12 indicates the clarity of thebright region formed by the concentric circle closest to the centerpoint in the 2D Fourier transform images of FIG. 38. As the value of thepeak width L is smaller, the extent of the region formed by theconcentric circles is also smaller. Accordingly, the TEM photograph inFIG. 37 before the 2D Fourier transform exhibits a greater level ofregularity and orderliness. This is considered to correspond to how theproportion of the area occupied by the nanocrystal structure in the TEMphotograph is larger. Conversely, as the value of the peak width L islarger, the extent of the region formed by the concentric circles isalso larger. Accordingly, the TEM photograph in FIG. 37 before the 2DFourier transform exhibits a lower level of regularity and orderliness.The decrease in regularity and orderliness the TEM photograph in FIG. 37before the 2D Fourier transform is considered to correspond to adecrease in the proportion of the area occupied by the nanocrystalstructure in the TEM photograph.

(Analysis of Relationship Between Nanocrystal Structure and HoleConduction Efficiency)

The experiments in embodiment 2 have revealed the following. A holeinjection layer with good hole conduction efficiency has the occupiedenergy level near the Fermi surface throughout the entire layer, and theproportion of tungsten atoms with a valence of five is high.Furthermore, the hole injection layer has the nanocrystal structure, andthe layer is regular and orderly. Conversely, in a hole injection layerwith poor hole conduction efficiency, the occupied energy level near theFermi surface could not be confirmed throughout the entire layer, andthe proportion of tungsten atoms with a valence of five is extremelylow. The nanocrystal structure could not be confirmed in such a holeinjection layer, and the structure of the layer was neither regular nororderly. The relation between these experimental results is analyzedbelow.

First, the relation between the nanocrystal structure (the regularity ofthe film structure) and the tungsten atoms with a valence of five isexamined.

As described above, in the hole injection layer formed under each filmforming condition in embodiment 2, the composition ratio of tungsten tooxygen is nearly 1:3. Accordingly, the nanocrystal structure thatprovides the layer structure with regularity as observed in the holeinjection layers under film forming conditions α, β, γ, and δ is assumedto be a microcrystal structure of tungsten trioxide.

Typically, when the oxygen vacancy structure is formed within ananoscale microcrystal, the region over which the oxygen vacancystructure exerts an influence is extremely large in relative terms, dueto the small size of the microcrystal. The microcrystal thus becomesgreatly distorted, making it difficult to maintain a crystal structure.Accordingly, tungsten atoms with a valence of five that derive from theoxygen vacancy or a similar structure are unlikely to be included withinthe nanocrystal.

This is not necessarily true, however, along a surface of a nanocrystal,nor at the grain boundaries between nanocrystals. Typically, a structuresimilar to the oxygen vacancy structure, known as a surface oxygenvacancy, easily forms on surfaces or grain boundaries where the crystalperiodicity is interrupted. For example, Non-Patent Literature 6 reportsthat the surface of tungsten trioxide crystals is more stable when halfof the tungsten atoms along the outermost surface do not terminate inoxygen atoms than when all of the tungsten atoms along the outermostsurface terminate in oxygen atoms. In this way, it is considered that alarge number of tungsten atoms with a valence of five that do notterminate in oxygen atoms exist along the surface and grain boundariesof the nanocrystals.

On the other hand, in the hole injection layer under film formingcondition ε, almost no tungsten atoms with a valence of five arepresent, and the nanocrystal structure was not confirmed. The entirelayer had the amorphous structure, with little regularity. This isconsidered to be because while the octahedrons that represent the basicstructure of tungsten trioxide share an oxygen atom at the verticeswithout interruption (and therefore do not become tungsten atoms with avalence of five), the arrangement of the octahedrons lacks periodicityand order.

Next, the relation between the occupied energy level near the Fermisurface and the tungsten atoms with a valence of five is described.

It is considered that the occupied energy level near the Fermi surfacederives from the oxygen vacancy structure or a similar structure, asexplained in embodiment 1. It is also considered that tungsten atomswith a valance of five derive from the oxygen vacancy structure or asimilar structure. In other words, the occupied energy level near theFermi surface and the tungsten atoms with a valence of five both derivefrom the oxygen vacancy structure or a similar structure. Specifically,as already mentioned in embodiment 1, there have been many reports ofassumption being made that a 5d orbital of a tungsten atom with avalence of five, which is not used for bonding with an oxygen atom,corresponds to the occupied energy level near the Fermi surface.

Based on the above, it can be inferred that in a hole injection layerwith good hole conduction efficiency, a large number of tungsten atomswith a valence of five exists along the surface and grain boundaries ofthe nanocrystals. Therefore, along the surface and at the grainboundaries, the overlap between 5d orbitals of the tungsten atoms with avalence of five grows large, so that the occupied energy level near theFermi surface exists continuously. On the other hand, a hole injectionlayer with poor hole conduction efficiency is amorphous, having almostnone of the oxygen vacancy structures or similar structures nor tungstenatoms with a valence of five deriving from such structures, and thus itcan be inferred that no occupied energy level near the Fermi surfaceexists in the layer.

Next, the mechanism for hole conduction in the hole injection layerpertaining to the present invention is analyzed further. The conductionof holes between tungsten atoms with a valence of five in the holeinjection layer 4A has already been analyzed. Based on the correlationbetween the above experiment results, however, it is possible toextrapolate a more concrete image.

First, hole conduction is described in a hole injection layer composedof tungsten oxide mainly including the amorphous structure, as in thehole injection layer formed under film forming condition E. FIG. 43Bshows the conduction of holes 14 in a hole injection layer in which anamorphous structure 16 is dominant, with few (or no) nanocrystals 15present. Tungsten atoms with a valence of five are scattered throughoutthe amorphous structure 16, and upon the application of voltage to thehole injection layer, holes 14 hop between tungsten atoms with a valenceof five that are relatively close to each other. Receiving the force ofthe electrical field, the holes 14 move to the buffer layer by hoppingbetween tungsten atoms with a valence of five that are close to eachother. In other words, in the amorphous structure 16, the holes 14 movethrough hopping conduction.

When the number of tungsten atoms with a valence of five is extremelylow, as in the hole injection layer under film forming condition ε, thetungsten atoms with a valence of five are separated by a long distance.In order to hop across this long distance, an extremely high voltageneeds to be applied, thus raising the driving voltage of the element.

In order to avoid this increase in voltage, the number of tungsten atomswith a valence of five, and therefore the oxygen vacancy structures orsimilar structures, should be increased in the amorphous structure 16.It is in fact possible to manufacture an amorphous layer that includesmany of the oxygen vacancy structures or similar structures by formingthe tungsten oxide through, for example, vacuum deposition underpre-determined conditions.

Such an amorphous layer with many of the oxygen vacancy structures orsimilar structures, however, loses chemical stability. Furthermore, aclear discoloring occurs due to the absorption of light by the oxygenvacancy structures or similar structures. Therefore, this approach isnot practical for the mass production of organic EL display panels. Bycontrast, in the hole injection layer pertaining to the presentinvention, the composition ratio of tungsten to oxygen is nearly 1:3.Therefore, few of the oxygen vacancy structures or similar structuresare found when the layer is seen in entirety, and the layer has acrystalline structure. This hole injection layer thus exhibitsrelatively good chemical stability and less discoloring.

Note that many tungsten atoms with a valence of five exist along thesurface of the nanocrystals 15 in FIG. 43B. Therefore, the occupiedenergy levels near the Fermi surface, which allow for the exchange ofholes, exist along the surface of the nanocrystals 15. As a result, theholes 14 move easily at least at the surface of the nanocrystals 15. Inorder to reach the buffer layer, however, the holes 14 must traverse theamorphous structure 16, thus preventing the hole conduction efficiencyfrom improving.

Next, hole conduction within a tungsten oxide layer having thenanocrystal structure pertaining to the present invention is described.FIG. 43A shows the conduction of holes 14 in a hole injection layerincluding the amorphous structure 16 by a small extent or not includingthe amorphous structure, and including an abundance of the nanocrystals13. First, as described above, an abundance of the tungsten atoms with avalence of five exist along the surface and grain boundaries of thenanocrystals 13. Therefore, the occupied energy levels near the Fermisurface, which can exchange holes, nearly continuously exist along thesurface and grain boundaries. Furthermore, since an abundance ofnanocrystals 13 are present in FIG. 43A, the surfaces and grainboundaries of these nanocrystals 13 are also continuous. In other words,continuous hole conduction paths exist along the continuous surfaces andgrain boundaries of the nanocrystals 13, as indicated by the boldfacearrows. As a result, upon applying voltage to the hole injection layer,the holes 14 are easily conducted along the occupied energy levels nearthe Fermi surface that extend along these continuous surfaces and grainboundaries, thus allowing the holes 14 to reach the buffer layer at alow driving voltage.

Based on the above analysis, the important factors for achieving a metaloxide layer with good hole conduction efficiency are (i) the existenceof portions that can exchange holes, and (ii) continuity among theseportions. Accordingly, a metal oxide layer that (i) includes metal atomsat a lower valence than the maximum possible valence and that (ii) hasthe nanocrystal structure can be considered a good structure for holeconduction.

The experiments and the observations provided for embodiment 2 abovehave been described based on the hole injection layer 4A. However,needless to say, such experiments and analysis hold as well for the holeinjection layer 4B.

<Additional Matters>

The hole injection layer pertaining to the present invention is notlimited to being formed by reactive sputtering. For example, vapordeposition, the CVD, or the like may be used instead for forming thehole injection layer.

The organic EL element pertaining to the present invention is notlimited to being used as a single element. A plurality of the organic ELelements may be layered on a substrate as pixels in order to form anorganic EL light-emitting apparatus. Such an organic EL light-emittingapparatus can be practiced by appropriately setting the thickness ofeach layer in each element and may, for example, be used as anillumination device or the like. Alternatively, the organic ELlight-emitting apparatus may be implemented as an organic EL panel,which is an image display device.

In embodiment 2, the position at which the peak P1 in FIG. 42 begins torise is the position along the horizontal axis, in the direction of thecenter point from the top of the peak P1, at which the derivative firstbecomes zero in portions (a2) and (b2) in FIG. 42. The method ofdetermining the position at which the peak P1 begins to rise is not,however, limited in this way. For example, in portion (a1) in FIG. 42,the average value of the normalized luminance near the peak P1 may beused as a baseline, with the position at which the peak P1 begins torise being defined as the intersection of this baseline and the line inthe graph near the peak P1.

In the organic EL element pertaining to the present invention, a holetransport layer may be provided between the hole injection layer and thelight-emitting layer. The hole transport layer has the function oftransporting holes injected from the hole injection layer to thelight-emitting layer. An organic material with hole transportingproperties is used as the hole transport layer. Organic material havinghole-transporting characteristics as described above refers to organicmaterial having characteristics of conveying holes having been generatedby making use of a charge transfer effect occurring between molecules.This is also known as a p-type organic semiconductor.

The material for the hole transport layer may be either a high molecularmaterial or a low molecular material, and the hole transport layer maybe formed by wet printing, for example. Further, it is desirable for thematerial for the hole transport layer to include a cross-linking agent,so that the material for the hole transport layer does not mix with thematerial for the light-emitting layer in the forming of thelight-emitting layer, which is disposed above the hole transport layer.Examples of the material for the hole transport layer are a copolymerthat includes a fluorene region and a triarylamine region, and atriarylamine derivative with a low molecular weight. One example of thecrosslinking agent that may be utilized is dipentaerythritolhexaacrylate. In this case, it is desirable that the cross-linking agentbe formed from poly(3,4-ethylenedioxythiophene) doped with polystyrenesulfonic acid (PEDOT: PSS) or a derivative thereof (a copolymer or thelike).

In embodiment 2, the anode 2 is composed of a thin film of Au in theorganic EL element 1C. Due to this, the ITO layer 3 is formed above theanode 2 to enhance the bonding between layers. However, when forming theanode 2 by using a material mainly including Al, excellent bondingbetween layers is achieved, and thus the anode 2 may be formed to have asingle layer structure.

When manufacturing an organic EL panel by using the organic EL elementpertaining to the present invention, the shape formed by the bankstherein is not limited to the so-called pixel bank structure (agrid-pattern of banks), and the banks may form a so-called line-bankstructure. FIG. 44 illustrates a structure of an organic EL panel havinga plurality of line banks 65 separating light-emitting layers 66 a, 66b, and 66 c adjacent in the X-axis direction from one another. Whenadopting the line banks 65, although light-emitting layers adjacent inthe Y-axis direction are not individually defined by a bank element,such light-emitting layers can be caused to perform light-emissionwithout interfering one another if the drive method, the areas ofanodes, the intervals between anodes, etc., are appropriately set.

In embodiments 1 and 2, organic material is used as the bank material,but alternatively inorganic material may be used. In this case as well,the bank material film is formed by application or by another method, aswhen using organic material.

INDUSTRIAL APPLICABILITY

The organic EL element pertaining to the present invention is to be usedas display elements for mobile phone displays and TVs, and as a lightsource for various applications. Regardless of the specific use thereof,the organic EL element pertaining to the present invention is applicableas an organic EL element having a wide range of luminous intensity fromlow luminous intensity to high luminous intensity for the use as a lightsource or the like, and which can be driven at low voltage. The organicEL element pertaining to the present invention, for having such a highlevel of performance, may be used in a wide range of applications,including those for household use, those for use in public facilities,and those for professional use. More specifically, such applicationsinclude: various display devices; TV apparatuses; displays for portableelectronic devices; illumination light sources, and etc.

REFERENCE SIGNS LIST

-   -   1, 1C organic EL elements    -   1A sample for photoelectron spectroscopy measurement    -   1B, 1D hole-only devices    -   2 anode    -   3 ITO layer    -   3A IZO layer    -   4X thin film (tungsten oxide film)    -   4, 4A, 4B hole injection layers    -   5X bank material film    -   5 banks    -   6A buffer layer    -   6B light-emitting layer    -   8 cathode (including two layers)    -   8A barium layer (layer included in cathode)    -   8B aluminum layer (layer included in cathode)    -   8C, 8E cathode (including one layer of Au)    -   8D cathode (including one layer of ITO)    -   9 sealing layer    -   10 substrate    -   11 silicon substrate    -   12 tungsten oxide layer    -   13, 15 nanocrystal    -   14 hole    -   16 amorphous structure    -   17 planarizing film    -   DC direct current power supply

The invention claimed is:
 1. A method for manufacturing an organiclight-emitting element, the method comprising: forming a tungsten oxidelayer containing tungsten oxide on a base layer including an anode, thetungsten oxide layer formed to have an oxygen vacancy structure therein;firing the tungsten oxide layer; forming a film of barrier wall materialon the fired tungsten oxide layer; forming barrier walls by patterningthe film of barrier wall material in a predetermined pattern by using anetching solution, the barrier walls defining an aperture; forming anorganic layer within the aperture, the organic layer containing organicmaterial; and forming a cathode above the organic layer, wherein in thefiring of the tungsten oxide layer, the firing is performed to providethe fired tungsten oxide layer with higher dissolution resistance to theetching solution compared to the tungsten oxide layer, while ensuringthat the fired tungsten oxide layer has an oxygen vacancy structuretherein.
 2. The method of claim 1, wherein in the firing of the tungstenoxide layer, the firing is performed to provide the fired tungsten oxidelayer with a film density of at least 5.8 g/cm³ and at most 6.0 g/cm³.3. The method of claim 1, wherein in the forming of the tungsten oxidelayer, the tungsten oxide layer is formed to have an occupied energylevel within a binding energy range from approximately 1.8 electronvolts to approximately 3.6 electron volts lower than a lowest bindingenergy of a valence band, and in the firing of the tungsten oxide layer,the firing is performed while ensuring that the fired tungsten oxidelayer has an occupied energy level within the binding energy range. 4.The method of claim 1, wherein in the forming of the tungsten oxidelayer, the tungsten oxide layer is formed such that at least one of anultraviolet photoelectron spectroscopy spectrum and an X-rayphotoelectron spectroscopy spectrum of the tungsten oxide layer exhibitsan upward protrusion within a binding energy range from approximately1.8 electron volts to approximately 3.6 electron volts lower than alowest binding energy of a valence band, and in the firing of thetungsten oxide layer, the firing is performed while ensuring that atleast one of an ultraviolet photoelectron spectroscopy spectrum and anX-ray photoelectron spectroscopy spectrum of the fired tungsten oxidelayer exhibits an upward protrusion within the binding energy range. 5.The method of claim 1, wherein in the forming of the tungsten oxidelayer, the tungsten oxide layer is formed such that a differentialspectrum obtained by differentiating an ultraviolet photoelectronspectroscopy spectrum of the tungsten oxide layer has a shape that isexpressed by a non-exponential function throughout a binding energyrange from approximately 1.8 electron volts to approximately 3.6electron volts lower than a lowest binding energy of a valence band, andin the firing of the tungsten oxide layer, the firing is performed whileensuring that a differential spectrum obtained by differentiating anultraviolet photoelectron spectroscopy spectrum of the fired tungstenoxide layer has a shape that is expressed by a non-exponential functionthroughout the binding energy range.
 6. The method of claim 1, whereinin the forming of the tungsten oxide layer, the tungsten oxide layer isfondled to include tungsten atoms with a valence of six and tungstenatoms with a valence of five and thus have an oxygen vacancy structuretherein, a ratio W⁵⁺/W⁶⁺ of the number of the tungsten atoms with avalence of five to the number of the tungsten atoms with a valence ofsix being at least 3.2% and at most 7.4%, and in the firing of thetungsten oxide layer, the firing is performed while ensuring that theratio W⁵⁺/W⁶⁺ in the fired tungsten oxide layer is at least 3.2% and atmost 7.4%.
 7. A method for manufacturing an organic light-emittingelement, the method comprising: forming a tungsten oxide layer on a baselayer including an anode, the tungsten oxide layer containing tungstenoxide; firing the tungsten oxide layer to obtain a fired tungsten oxidelayer; forming a film of barrier wall material on the fired tungstenoxide layer; forming barrier walls by patterning the film of barrierwall material in a predetermined pattern by using an etching solution,the barrier walls defining an aperture; forming an organic layer withinthe aperture, the organic layer containing organic material; and forminga cathode above the organic layer, wherein in the forming of thetungsten oxide layer, the tungsten oxide layer is formed such that atleast one of an ultraviolet photoelectron spectroscopy spectrum and anX-ray photoelectron spectroscopy spectrum of the tungsten oxide layerexhibits an upward protrusion within a binding energy range fromapproximately 1.8 electron volts to approximately 3.6 electron voltslower than a lowest binding energy of a valence band, and in the firingof the tungsten oxide layer, the firing is performed to provide thefired tungsten oxide layer with higher dissolution resistance to theetching solution compared to the tungsten oxide layer, while ensuringthat at least one of an ultraviolet photoelectron spectroscopy spectrumand an X-ray photoelectron spectroscopy spectrum of the fired tungstenoxide layer exhibits an upward protrusion within the binding energyrange.
 8. A method for manufacturing an organic light-emitting element,the method comprising: forming a tungsten oxide layer on a base layerincluding an anode, the tungsten oxide layer containing tungsten oxide;firing the tungsten oxide layer to obtain a fired tungsten oxide layer;forming a film of barrier wall material on the fired tungsten oxidelayer; forming barrier walls by patterning the film of barrier wallmaterial in a predetermined pattern by using an etching solution, thebarrier walls defining an aperture; forming an organic layer within theaperture, the organic layer containing organic material; and forming acathode above the organic layer, wherein in the forming of the tungstenoxide layer, the tungsten oxide layer is formed to include tungstenatoms with a valence of six and tungsten atoms with a valence of fiveand thus have an oxygen vacancy structure therein, a ratio W⁵⁺/W⁶⁺ ofthe number of the tungsten atoms with a valence of five to the number ofthe tungsten atoms with a valence of six being at least 3.2% and at most7.4%, and in the firing of the tungsten oxide layer, the firing isperformed to provide the fired tungsten oxide layer with higherdissolution resistance to the etching solution compared to the tungstenoxide layer, while ensuring that the ratio W⁵⁺/W⁶⁺ in the fired tungstenoxide layer is at least 3.2% and at most 7.4%.